diff --git a/src/lib/organize.js b/src/lib/organize.js
new file mode 100644
index 0000000..564d1e3
--- /dev/null
+++ b/src/lib/organize.js
@@ -0,0 +1,154 @@
+/**
+ * GCD-based recurrence detection - port of ical_organizer.py.
+ * Collapses a flat list of per-occurrence events into RRULE + EXDATE.
+ */
+import { bydayFromDate } from './weekday.js'
+
+// ------------------------------------------------------------------ //
+// Grouping
+// ------------------------------------------------------------------ //
+/**
+ * Identity of a recurring series.
+ * Two events belong together iff they share summary, location, start
+ * wall-clock time, duration (minutes), and weekday.
+ */
+export function seriesKey(ev) {
+  const start = new Date(ev.dtstartDate + 'T' + ev.dtstartTime + ':00')
+  const [sh, sm] = ev.dtstartTime.split(':').map(Number)
+  const [eh, em] = ev.dtendTime.split(':').map(Number)
+  const durationMin = (eh * 60 + em) - (sh * 60 + sm)
+  return [ev.summary, ev.location, ev.dtstartTime, durationMin, start.getDay()].join('|')
+}
+
+// ------------------------------------------------------------------ //
+// Interval detection
+// ------------------------------------------------------------------ //
+function gcd(a, b) {
+  while (b) { [a, b] = [b, a % b] }
+  return a
+}
+
+/**
+ * Return the recurrence interval in days, or null if irregular.
+ * Computes GCD of all consecutive date gaps.
+ */
+export function detectInterval(dateStrings) {
+  if (dateStrings.length < 2) return null
+  const dates = [...dateStrings].sort()
+  const gaps = []
+  for (let i = 0; i < dates.length - 1; i++) {
+    const a = new Date(dates[i] + 'T00:00:00')
+    const b = new Date(dates[i + 1] + 'T00:00:00')
+    gaps.push(Math.round((b - a) / (24 * 3600 * 1000)))
+  }
+  let g = gaps[0]
+  for (let i = 1; i < gaps.length; i++) {
+    g = gcd(g, gaps[i])
+  }
+  return g > 0 ? g : null
+}
+
+// ------------------------------------------------------------------ //
+// Series fitting
+// ------------------------------------------------------------------ //
+/**
+ * Try to collapse events into one recurring event.
+ * Returns { base, rrule, exdates } or null to decline.
+ */
+export function fitSeries(evs) {
+  if (evs.length < 2) return null
+
+  const sorted = [...evs].sort((a, b) =>
+    a.dtstartDate < b.dtstartDate ? -1 : a.dtstartDate > b.dtstartDate ? 1 : 0,
+  )
+  const starts = sorted.map(e => e.dtstartDate)
+  const interval = detectInterval(starts)
+  if (!interval) return null
+
+  const first = sorted[0]
+  const last = sorted[sorted.length - 1]
+  const present = new Set(starts)
+
+  // Build expected grid first..last step interval days
+  const expected = []
+  const cur = new Date(first.dtstartDate + 'T00:00:00')
+  const lastDate = new Date(last.dtstartDate + 'T00:00:00')
+  while (cur <= lastDate) {
+    const y = cur.getFullYear()
+    const m = String(cur.getMonth() + 1).padStart(2, '0')
+    const d = String(cur.getDate()).padStart(2, '0')
+    expected.push(`${y}-${m}-${d}`)
+    cur.setDate(cur.getDate() + interval)
+  }
+
+  const expectedSet = new Set(expected)
+  const missing = expected.filter(d => !present.has(d))
+  const offGrid = starts.filter(d => !expectedSet.has(d))
+  if (offGrid.length > 0) return null
+
+  // "Mostly regular": at least 2 occurrences and occurrences >= skips
+  if (evs.length < 2 || missing.length >= evs.length) return null
+
+  // Build rrule
+  const firstDate = new Date(first.dtstartDate + 'T00:00:00')
+  let freq, step, byday
+  if (interval % 7 === 0) {
+    freq = 'WEEKLY'
+    step = interval / 7
+    byday = bydayFromDate(firstDate)
+  } else {
+    freq = 'DAILY'
+    step = interval
+    byday = null
+  }
+
+  const rrule = { freq, interval: step, byday, untilDate: last.dtstartDate }
+  return { base: first, rrule, exdates: missing }
+}
+
+// ------------------------------------------------------------------ //
+// Organize
+// ------------------------------------------------------------------ //
+/**
+ * Collapse flat events into recurring series.
+ * Returns { events, stats: { series, flat } }.
+ */
+export function organize(events) {
+  // Group by series key
+  const groups = new Map()
+  for (const ev of events) {
+    const key = seriesKey(ev)
+    if (!groups.has(key)) groups.set(key, [])
+    groups.get(key).push(ev)
+  }
+
+  const result = []
+  let nSeries = 0
+  let nFlat = 0
+
+  for (const evs of groups.values()) {
+    // Sort by date
+    evs.sort((a, b) =>
+      a.dtstartDate < b.dtstartDate ? -1 : a.dtstartDate > b.dtstartDate ? 1 : 0,
+    )
+
+    const fit = fitSeries(evs)
+    if (fit) {
+      // Collapsed event: preserve base's UID (not reassign)
+      result.push({
+        ...fit.base,
+        rrule: fit.rrule,
+        exdates: fit.exdates,
+      })
+      nSeries++
+    } else {
+      // Keep flat
+      for (const ev of evs) {
+        result.push(ev)
+        nFlat++
+      }
+    }
+  }
+
+  return { events: result, stats: { series: nSeries, flat: nFlat } }
+}
diff --git a/test/organize.test.js b/test/organize.test.js
new file mode 100644
index 0000000..3388d1e
--- /dev/null
+++ b/test/organize.test.js
@@ -0,0 +1,100 @@
+import { describe, it, expect } from 'vitest'
+import { readFileSync } from 'node:fs'
+import { fileURLToPath } from 'node:url'
+import { dirname, join } from 'node:path'
+import { parseICS } from '../src/lib/ical-io.js'
+import { organize, seriesKey, detectInterval, fitSeries } from '../src/lib/organize.js'
+
+const __dirname = dirname(fileURLToPath(import.meta.url))
+const GOLDEN = readFileSync(join(__dirname, 'golden-org.ics'), 'utf-8')
+const { events } = parseICS(GOLDEN)
+
+describe('seriesKey', () => {
+  it('groups events with same summary/location/time/duration/weekday', () => {
+    const e1 = events[0] // ACC INFO SYS Tutorial, Mon 18:00
+    const e2 = events[1] // same series, next week
+    expect(seriesKey(e1)).toBe(seriesKey(e2))
+  })
+
+  it('separates events with different summary', () => {
+    const e1 = events[0] // ACC INFO SYS
+    // Find a BUS LAW event
+    const eBus = events.find(e => e.summary.startsWith('BUS LAW, Tutorial'))
+    expect(seriesKey(e1)).not.toBe(seriesKey(eBus))
+  })
+})
+
+describe('detectInterval', () => {
+  it('returns 7 for weekly dates', () => {
+    const dates = ['2026-07-27', '2026-08-03', '2026-08-10', '2026-08-17']
+    expect(detectInterval(dates)).toBe(7)
+  })
+
+  it('returns null for single date', () => {
+    expect(detectInterval(['2026-07-27'])).toBeNull()
+  })
+
+  it('returns 7 for dates with a skip (gaps 7,7,14,7)', () => {
+    const dates = ['2026-07-27', '2026-08-03', '2026-08-10', '2026-08-24', '2026-08-31']
+    expect(detectInterval(dates)).toBe(7)
+  })
+})
+
+describe('fitSeries', () => {
+  it('fits ACC INFO SYS series (12 events, 1 skip)', () => {
+    const accEvents = events.filter(e => e.summary === 'ACC INFO SYS, Tutorial')
+    const result = fitSeries(accEvents)
+    expect(result).not.toBeNull()
+    expect(result.rrule.freq).toBe('WEEKLY')
+    expect(result.rrule.interval).toBe(1)
+    expect(result.rrule.byday).toBe('MO')
+    expect(result.exdates).toContain('2026-09-21')
+  })
+
+  it('rejects BUS LAW Workshop (too sparse: 3 events, 5 missing)', () => {
+    const wsEvents = events.filter(e => e.summary === 'BUS LAW, Workshop')
+    expect(wsEvents).toHaveLength(3)
+    expect(fitSeries(wsEvents)).toBeNull()
+  })
+})
+
+describe('organize (golden test against org.ics)', () => {
+  const { events: organized, stats } = organize(events)
+
+  it('collapses 61 events into 8', () => {
+    expect(organized).toHaveLength(8)
+    expect(stats.series).toBe(5)
+    expect(stats.flat).toBe(3)
+  })
+
+  it('produces 5 recurring events with RRULE', () => {
+    const recurring = organized.filter(e => e.rrule !== null)
+    expect(recurring).toHaveLength(5)
+  })
+
+  it('produces 3 flat BUS LAW Workshop events', () => {
+    const flatWs = organized.filter(
+      e => e.summary === 'BUS LAW, Workshop' && e.rrule === null,
+    )
+    expect(flatWs).toHaveLength(3)
+  })
+
+  it('ACC INFO SYS series has 1 exdate (Sep 21)', () => {
+    const acc = organized.find(e => e.summary === 'ACC INFO SYS, Tutorial')
+    expect(acc.rrule).not.toBeNull()
+    expect(acc.exdates).toEqual(['2026-09-21'])
+  })
+
+  it('BUS LAW Tutorial series has 2 exdates', () => {
+    const bl = organized.find(e => e.summary === 'BUS LAW, Tutorial')
+    expect(bl.rrule).not.toBeNull()
+    expect(bl.exdates).toHaveLength(2)
+    expect(bl.exdates).toContain('2026-08-31')
+    expect(bl.exdates).toContain('2026-09-21')
+  })
+
+  it('preserves original UID of base event', () => {
+    const acc = organized.find(e => e.summary === 'ACC INFO SYS, Tutorial')
+    expect(acc.uid).toBe('uid0') // first event in series
+  })
+})