diff --git a/apps/client/package.json b/apps/client/package.json
index 9db5093b45..1fd8bea2c6 100644
--- a/apps/client/package.json
+++ b/apps/client/package.json
@@ -22,8 +22,8 @@
     "@fullcalendar/interaction": "6.1.20",
     "@fullcalendar/list": "6.1.20",
     "@fullcalendar/multimonth": "6.1.20",
-    "@fullcalendar/timegrid": "6.1.20",
     "@fullcalendar/rrule": "6.1.20",
+    "@fullcalendar/timegrid": "6.1.20",
     "@maplibre/maplibre-gl-leaflet": "0.1.3",
     "@mermaid-js/layout-elk": "0.2.0",
     "@mind-elixir/node-menu": "5.0.1",
@@ -64,6 +64,7 @@
     "react-i18next": "16.5.4",
     "react-window": "2.2.7",
     "reveal.js": "5.2.1",
+    "rrule": "2.8.1",
     "svg-pan-zoom": "3.6.2",
     "tabulator-tables": "6.3.1",
     "vanilla-js-wheel-zoom": "9.0.4"
diff --git a/apps/client/src/widgets/collections/calendar/event_builder.ts b/apps/client/src/widgets/collections/calendar/event_builder.ts
index 4967a5ca1b..b60a203d89 100644
--- a/apps/client/src/widgets/collections/calendar/event_builder.ts
+++ b/apps/client/src/widgets/collections/calendar/event_builder.ts
@@ -1,9 +1,11 @@
 import { EventInput, EventSourceFuncArg, EventSourceInput } from "@fullcalendar/core/index.js";
 import clsx from "clsx";
+import * as rruleLib from 'rrule';
 
 import FNote from "../../../entities/fnote";
 import froca from "../../../services/froca";
 import server from "../../../services/server";
+import toastService from "../../../services/toast";
 import { formatDateToLocalISO, getCustomisableLabel, getMonthsInDateRange, offsetDate } from "./utils";
 
 interface Event {
@@ -122,13 +124,30 @@ export async function buildEvent(note: FNote, { startDate, endDate, startTime, e
         }
 
         if (recurrence) {
-            delete eventData.end;
-            eventData.rrule = `DTSTART:${startDate.replace(/[-:]/g, "")}\n${recurrence}`;
-            if (endDate){
-                const diffMs = new Date(endDate).getTime() - new Date(startDate).getTime();
-                const hours = Math.floor(diffMs / 3600000);
-                const minutes = Math.floor((diffMs / 60000) % 60);
-                eventData.duration = `${String(hours).padStart(2, "0")}:${String(minutes).padStart(2, "0")}`;
+            // Generate rrule string
+            const rruleString = `DTSTART:${startDate.replace(/[-:]/g, "")}\n${recurrence}`;
+
+            // Validate rrule string
+            let rruleValid = true;
+            try {
+                rruleLib.rrulestr(rruleString, { forceset: true }) as rruleLib.RRuleSet;
+            } catch {
+                rruleValid = false;
+            }
+
+            if (rruleValid) {
+                delete eventData.end;
+                eventData.rrule = rruleString;
+                if (endDate){
+                    const diffMs = new Date(endDate).getTime() - new Date(startDate).getTime();
+                    const hours = Math.floor(diffMs / 3600000);
+                    const minutes = Math.floor((diffMs / 60000) % 60);
+                    eventData.duration = `${String(hours).padStart(2, "0")}:${String(minutes).padStart(2, "0")}`;
+                }
+            } else {
+                const errorMessage = `Note "${note.noteId} ${note.title}" has an invalid #recurrence string. This note will not recur.`;
+                toastService.showError(errorMessage);
+                console.error(errorMessage);
             }
         }
         events.push(eventData);
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 6b1e2d6e85..0097f4aaf2 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -326,6 +326,9 @@ importers:
       reveal.js:
         specifier: 5.2.1
         version: 5.2.1
+      rrule:
+        specifier: 2.8.1
+        version: 2.8.1
       svg-pan-zoom:
         specifier: 3.6.2
         version: 3.6.2