diff --git a/lib/events/calendar.sx b/lib/events/calendar.sx
index cb32e7ae..cc65f7ff 100644
--- a/lib/events/calendar.sx
+++ b/lib/events/calendar.sx
@@ -9,8 +9,9 @@
 ;; bounded list of occurrence dicts within an explicit (win-start, win-end)
 ;; window. Expansion is ALWAYS windowed — an RRULE without a window is an
 ;; infinite computation and is never permitted. Supported subset (RFC 5545):
-;; FREQ=DAILY|WEEKLY, INTERVAL, COUNT, UNTIL, BYDAY (weekly). MONTHLY and the
-;; rest are deferred.
+;; FREQ=DAILY|WEEKLY|MONTHLY, INTERVAL, COUNT, UNTIL, BYDAY (weekly: weekday
+;; numbers; monthly: {:ord N :wd W} ordinal weekdays), BYMONTHDAY (monthly,
+;; negative = from month end). YEARLY and the rest are deferred.
 
 ;; ---- integer helpers ----
 
@@ -20,6 +21,8 @@
 
 (define ev-or (fn (x d) (if (nil? x) d x)))
 
+(define ev-filter-nil (fn (xs) (filter (fn (x) (not (nil? x))) xs)))
+
 ;; ---- civil date core (Hinnant) ----
 
 ;; Days since 1970-01-01 for civil (y, m, d). m in [1,12], d in [1,31].
@@ -83,6 +86,28 @@
 ;; Weekday of a day-number: 0=Mon .. 6=Sun (1970-01-01 is Thursday = 3).
 (define ev-weekday-of-days (fn (z) (modulo (+ z 3) 7)))
 
+(define
+  ev-days-in-month
+  (fn
+    (y m)
+    (-
+      (ev-days-from-civil
+        (if (= m 12) (+ y 1) y)
+        (if (= m 12) 1 (+ m 1))
+        1)
+      (ev-days-from-civil y m 1))))
+
+;; Add k months to (y,m), returning (list y2 m2).
+(define
+  ev-add-months
+  (fn
+    (y m k)
+    (let
+      ((total (+ (* y 12) (- m 1) k)))
+      (list
+        (ev-floor-div total 12)
+        (+ (modulo total 12) 1)))))
+
 ;; ---- datetime (epoch minutes) ----
 
 (define
@@ -101,11 +126,18 @@
 
 (define ev-dt-tod (fn (t) (modulo t 1440)))
 
+(define ev-civ-y (fn (c) (first c)))
+(define ev-civ-m (fn (c) (first (rest c))))
+(define ev-civ-d (fn (c) (first (rest (rest c)))))
+
 ;; ---- event + occurrence constructors ----
 
 ;; rrule is nil (single event) or a dict:
-;;   {:freq :daily|:weekly :interval N :count N|nil :until DT|nil
-;;    :byday (list 0 2 4)|nil}   ; byday weekday numbers, 0=Mon
+;;   {:freq :daily|:weekly|:monthly :interval N :count N|nil :until DT|nil
+;;    :byday ...|nil :bymonthday (list 15 -1)|nil}
+;;   weekly  :byday -> (list 0 2 4)            weekday numbers, 0=Mon
+;;   monthly :byday -> (list {:ord 2 :wd 1})   nth weekday (ord<0 from end)
+;;   monthly :bymonthday -> (list 15 -1)       day of month (negative from end)
 (define ev-event (fn (id dtstart duration rrule capacity) {:duration duration :id id :dtstart dtstart :capacity capacity :rrule rrule}))
 
 (define ev-occ (fn (id start dur) {:id id :start start :end (+ start dur)}))
@@ -138,10 +170,40 @@
             acc
             (+ n 1)))))))
 
+;; ---- shared per-period emit ----
+;; Walk a start-ascending list of candidate occurrence datetimes for one
+;; period, generating (count toward COUNT) those >= dtstart within UNTIL, and
+;; emitting those also inside the window. Returns the updated running n.
+(define
+  ev-emit-occs
+  (fn
+    (id occs duration count until dtstart win-start win-end acc n)
+    (if
+      (empty? occs)
+      n
+      (let
+        ((occ (first occs)))
+        (let
+          ((generates? (and (>= occ dtstart) (or (nil? until) (<= occ until)) (or (nil? count) (< n count)))))
+          (begin
+            (when
+              (and generates? (>= occ win-start) (<= occ win-end))
+              (append! acc (ev-occ id occ duration)))
+            (ev-emit-occs
+              id
+              (rest occs)
+              duration
+              count
+              until
+              dtstart
+              win-start
+              win-end
+              acc
+              (if generates? (+ n 1) n))))))))
+
 ;; ---- WEEKLY expansion ----
-;; Iterate week by week from the Monday of dtstart's week. Within each active
-;; week emit each BYDAY (sorted) whose datetime is >= dtstart and inside the
-;; window. n counts every generated occurrence >= dtstart for COUNT.
+;; Iterate week by week from the Monday of dtstart's week; within each active
+;; week emit each BYDAY (sorted). n counts every generated occurrence.
 
 (define
   ev-week0-days
@@ -153,37 +215,6 @@
     (byday dtstart)
     (if (nil? byday) (list (ev-dt-weekday dtstart)) (sort byday))))
 
-(define
-  ev-weekly-bd-loop
-  (fn
-    (id week-days tod duration bd count until dtstart win-start win-end acc n)
-    (if
-      (empty? bd)
-      n
-      (let
-        ((wd (first bd)))
-        (let
-          ((occ (+ (* (+ week-days wd) 1440) tod)))
-          (let
-            ((generates? (and (>= occ dtstart) (or (nil? until) (<= occ until)) (or (nil? count) (< n count)))))
-            (begin
-              (when
-                (and generates? (>= occ win-start) (<= occ win-end))
-                (append! acc (ev-occ id occ duration)))
-              (ev-weekly-bd-loop
-                id
-                week-days
-                tod
-                duration
-                (rest bd)
-                count
-                until
-                dtstart
-                win-start
-                win-end
-                acc
-                (if generates? (+ n 1) n)))))))))
-
 (define
   ev-weekly-loop
   (fn
@@ -207,21 +238,126 @@
         ((and (not (nil? count)) (>= n count)) acc)
         (else
           (let
-            ((n2 (ev-weekly-bd-loop id week-days tod duration bd count until dtstart win-start win-end acc n)))
-            (ev-weekly-loop
-              id
-              (+ week-days week-step)
-              tod
-              duration
-              week-step
-              bd
-              count
-              until
-              dtstart
-              win-start
-              win-end
-              acc
-              n2)))))))
+            ((occs (map (fn (wd) (+ (* (+ week-days wd) 1440) tod)) bd)))
+            (let
+              ((n2 (ev-emit-occs id occs duration count until dtstart win-start win-end acc n)))
+              (ev-weekly-loop
+                id
+                (+ week-days week-step)
+                tod
+                duration
+                week-step
+                bd
+                count
+                until
+                dtstart
+                win-start
+                win-end
+                acc
+                n2))))))))
+
+;; ---- MONTHLY expansion ----
+;; Iterate month by month from dtstart's month, stepping by INTERVAL months.
+;; Candidate days per month come from BYMONTHDAY, then ordinal BYDAY, else the
+;; day-of-month of dtstart (skipped in months too short to contain it).
+
+;; Resolve a BYMONTHDAY value to a valid day-of-month, or nil.
+(define
+  ev-resolve-monthday
+  (fn
+    (y m bmd)
+    (let
+      ((dim (ev-days-in-month y m)))
+      (let
+        ((day (if (< bmd 0) (+ dim 1 bmd) bmd)))
+        (if (and (>= day 1) (<= day dim)) day nil)))))
+
+;; Resolve an ordinal weekday {:ord :wd} to a day-of-month, or nil.
+(define
+  ev-resolve-nth-weekday
+  (fn
+    (y m ord wd)
+    (let
+      ((dim (ev-days-in-month y m)))
+      (if
+        (> ord 0)
+        (let
+          ((first-wd (ev-weekday-of-days (ev-days-from-civil y m 1))))
+          (let
+            ((day (+ 1 (modulo (- wd first-wd) 7) (* (- ord 1) 7))))
+            (if (<= day dim) day nil)))
+        (let
+          ((last-wd (ev-weekday-of-days (ev-days-from-civil y m dim))))
+          (let
+            ((day (- dim (modulo (- last-wd wd) 7) (* (- (- ord) 1) 7))))
+            (if (>= day 1) day nil)))))))
+
+(define
+  ev-month-candidates
+  (fn
+    (y m rrule dtstart)
+    (let
+      ((bmd (get rrule :bymonthday)) (byday (get rrule :byday)))
+      (cond
+        ((not (nil? bmd))
+          (ev-filter-nil (map (fn (d) (ev-resolve-monthday y m d)) bmd)))
+        ((not (nil? byday))
+          (ev-filter-nil
+            (map
+              (fn
+                (e)
+                (ev-resolve-nth-weekday y m (get e :ord) (get e :wd)))
+              byday)))
+        (else
+          (ev-filter-nil
+            (list
+              (ev-resolve-monthday y m (ev-civ-d (ev-dt->civil dtstart))))))))))
+
+(define
+  ev-monthly-loop
+  (fn
+    (id
+      y
+      m
+      rrule
+      duration
+      tod
+      interval
+      count
+      until
+      dtstart
+      win-start
+      win-end
+      acc
+      n)
+    (let
+      ((month-start (ev-dt y m 1 0 0)))
+      (cond
+        ((> month-start win-end) acc)
+        ((and (not (nil? count)) (>= n count)) acc)
+        (else
+          (let
+            ((days (sort (ev-month-candidates y m rrule dtstart))))
+            (let
+              ((occs (map (fn (d) (+ (* (ev-days-from-civil y m d) 1440) tod)) days)))
+              (let
+                ((n2 (ev-emit-occs id occs duration count until dtstart win-start win-end acc n))
+                  (nm (ev-add-months y m interval)))
+                (ev-monthly-loop
+                  id
+                  (ev-civ-y nm)
+                  (ev-civ-m nm)
+                  rrule
+                  duration
+                  tod
+                  interval
+                  count
+                  until
+                  dtstart
+                  win-start
+                  win-end
+                  acc
+                  n2)))))))))
 
 ;; ---- top-level expansion ----
 ;; Returns a list of occurrence dicts {:id :start :end} within the window.
@@ -277,6 +413,24 @@
                   win-end
                   acc
                   0))
+              ((= freq :monthly)
+                (let
+                  ((civ (ev-dt->civil dtstart)))
+                  (ev-monthly-loop
+                    id
+                    (ev-civ-y civ)
+                    (ev-civ-m civ)
+                    rrule
+                    duration
+                    (ev-dt-tod dtstart)
+                    interval
+                    count
+                    until
+                    dtstart
+                    win-start
+                    win-end
+                    acc
+                    0)))
               (else (error (str "ev-expand: unsupported freq: " freq))))
             acc))))))
 
diff --git a/lib/events/scoreboard.json b/lib/events/scoreboard.json
index 7e6b990a..a2bba6e6 100644
--- a/lib/events/scoreboard.json
+++ b/lib/events/scoreboard.json
@@ -1,10 +1,10 @@
 {
   "lang": "events",
-  "total_passed": 24,
+  "total_passed": 37,
   "total_failed": 0,
-  "total": 24,
+  "total": 37,
   "suites": [
-    {"name":"calendar","passed":24,"failed":0,"total":24}
+    {"name":"calendar","passed":37,"failed":0,"total":37}
   ],
-  "generated": "2026-06-06T23:48:10+00:00"
+  "generated": "2026-06-06T23:52:14+00:00"
 }
diff --git a/lib/events/scoreboard.md b/lib/events/scoreboard.md
index 331a9355..a446cdf6 100644
--- a/lib/events/scoreboard.md
+++ b/lib/events/scoreboard.md
@@ -1,7 +1,7 @@
 # events scoreboard
 
-**24 / 24 passing** (0 failure(s)).
+**37 / 37 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
-| calendar | 24 | 24 | ok |
+| calendar | 37 | 37 | ok |
diff --git a/lib/events/tests/calendar.sx b/lib/events/tests/calendar.sx
index 59f82476..e679398e 100644
--- a/lib/events/tests/calendar.sx
+++ b/lib/events/tests/calendar.sx
@@ -73,6 +73,22 @@
         (ev-dt->civil
           (ev-dt 2026 12 25 8 0))
         (list 2026 12 25))
+      (ev-cal-check!
+        "days in feb 2024 (leap)"
+        (ev-days-in-month 2024 2)
+        29)
+      (ev-cal-check!
+        "days in feb 2026"
+        (ev-days-in-month 2026 2)
+        28)
+      (ev-cal-check!
+        "add months wraps year"
+        (ev-add-months 2026 11 3)
+        (list 2027 2))
+      (ev-cal-check!
+        "add months within year"
+        (ev-add-months 2026 1 5)
+        (list 2026 6))
       (let
         ((ev (ev-event (quote one) (ev-dt 2026 6 10 14 0) 60 nil 1)))
         (do
@@ -264,6 +280,124 @@
             (list 2026 6 5)
             (list 2026 6 8)
             (list 2026 6 10))))
+      (let
+        ((md (ev-event (quote md) (ev-dt 2026 1 15 9 0) 60 {:bymonthday (list 15) :freq :monthly} 1)))
+        (do
+          (ev-cal-check!
+            "monthly bymonthday 15th"
+            (ev-cal-starts
+              (ev-expand
+                md
+                (ev-date 2026 1 1)
+                (ev-date 2026 4 1)))
+            (list
+              (list 2026 1 15)
+              (list 2026 2 15)
+              (list 2026 3 15)))
+          (ev-cal-check!
+            "monthly preserves time of day"
+            (ev-dt-tod
+              (get
+                (first
+                  (ev-expand
+                    md
+                    (ev-date 2026 1 1)
+                    (ev-date 2026 4 1)))
+                :start))
+            540)))
+      (let
+        ((mm (ev-event (quote mm) (ev-dt 2026 1 1 9 0) 60 {:bymonthday (list 1 15) :freq :monthly :count 4} 1)))
+        (ev-cal-check!
+          "monthly multiple bymonthday sorted within month"
+          (ev-cal-starts
+            (ev-expand
+              mm
+              (ev-date 2026 1 1)
+              (ev-date 2026 12 1)))
+          (list
+            (list 2026 1 1)
+            (list 2026 1 15)
+            (list 2026 2 1)
+            (list 2026 2 15))))
+      (let
+        ((ml (ev-event (quote ml) (ev-dt 2026 1 31 9 0) 60 {:bymonthday (list -1) :freq :monthly} 1)))
+        (ev-cal-check!
+          "monthly bymonthday -1 is last day"
+          (ev-cal-starts
+            (ev-expand
+              ml
+              (ev-date 2026 1 1)
+              (ev-date 2026 4 1)))
+          (list
+            (list 2026 1 31)
+            (list 2026 2 28)
+            (list 2026 3 31))))
+      (let
+        ((mn (ev-event (quote mn) (ev-dt 2026 1 1 9 0) 60 {:freq :monthly :byday (list {:ord 2 :wd 1})} 1)))
+        (ev-cal-check!
+          "monthly 2nd tuesday"
+          (ev-cal-shape
+            (ev-expand
+              mn
+              (ev-date 2026 1 1)
+              (ev-date 2026 4 1)))
+          (list
+            (list (list 2026 1 13) 1)
+            (list (list 2026 2 10) 1)
+            (list (list 2026 3 10) 1))))
+      (let
+        ((mz (ev-event (quote mz) (ev-dt 2026 1 1 9 0) 60 {:freq :monthly :byday (list {:ord -1 :wd 4})} 1)))
+        (ev-cal-check!
+          "monthly last friday"
+          (ev-cal-shape
+            (ev-expand
+              mz
+              (ev-date 2026 1 1)
+              (ev-date 2026 4 1)))
+          (list
+            (list (list 2026 1 30) 4)
+            (list (list 2026 2 27) 4)
+            (list (list 2026 3 27) 4))))
+      (let
+        ((m31 (ev-event (quote m31) (ev-dt 2026 1 31 9 0) 60 {:freq :monthly :count 4} 1)))
+        (ev-cal-check!
+          "monthly default day-of-month skips short months"
+          (ev-cal-starts
+            (ev-expand
+              m31
+              (ev-date 2026 1 1)
+              (ev-date 2026 12 1)))
+          (list
+            (list 2026 1 31)
+            (list 2026 3 31)
+            (list 2026 5 31)
+            (list 2026 7 31))))
+      (let
+        ((mi (ev-event (quote mi) (ev-dt 2026 1 10 9 0) 60 {:interval 3 :freq :monthly :count 3} 1)))
+        (ev-cal-check!
+          "monthly interval 3 steps by quarter"
+          (ev-cal-starts
+            (ev-expand
+              mi
+              (ev-date 2026 1 1)
+              (ev-date 2027 1 1)))
+          (list
+            (list 2026 1 10)
+            (list 2026 4 10)
+            (list 2026 7 10))))
+      (let
+        ((mc (ev-event (quote mc) (ev-dt 2026 1 5 9 0) 60 {:freq :monthly :count 12} 1)))
+        (ev-cal-check!
+          "monthly count window-independent (clip middle)"
+          (ev-cal-starts
+            (ev-expand
+              mc
+              (ev-date 2026 4 1)
+              (ev-date 2026 6 30)))
+          (list
+            (list 2026 4 5)
+            (list 2026 5 5)
+            (list 2026 6 5))))
       (let
         ((a (ev-event (quote a) (ev-dt 2026 6 2 10 0) 30 {:freq :daily :count 2} 1))
           (b
diff --git a/plans/events-on-sx.md b/plans/events-on-sx.md
index a831c535..907aac89 100644
--- a/plans/events-on-sx.md
+++ b/plans/events-on-sx.md
@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **24/24** (Phase 1: calendar recurrence)
+`bash lib/events/conformance.sh` → **37/37** (Phase 1: calendar recurrence)
 
 ## Ground rules
 
@@ -55,7 +55,7 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Phase 1 — Calendar + recurrence
 - [x] `calendar.sx` — event facts, RRULE expansion in a window (DAILY/WEEKLY)
-- [ ] `calendar.sx` — MONTHLY (bymonthday + nth-weekday byday)
+- [x] `calendar.sx` — MONTHLY (bymonthday + nth-weekday byday)
 - [ ] `availability.sx` — free/busy rules
 - [ ] `api.sx` + tests + scoreboard + conformance.sh
 
@@ -77,6 +77,11 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-06 — MONTHLY recurrence. `ev-days-in-month`, `ev-add-months`,
+  BYMONTHDAY (incl. negative = from month end), ordinal BYDAY (`{:ord N :wd W}`,
+  ord<0 = nth-from-last), default day-of-month (skips months too short, e.g.
+  day-31 monthly skips Feb/Apr). Refactored weekly+monthly onto a shared
+  `ev-emit-occs` per-period emitter. 37/37 green (+13).
 - 2026-06-06 — Phase 1 scaffold + calendar recurrence. `calendar.sx`: integer
   epoch-minute datetimes, Hinnant civil<->day-number conversion, DAILY/WEEKLY
   RRULE expansion in a bounded (start,end) window with INTERVAL, COUNT (window-