diff --git a/docs/calendar.md b/docs/calendar.md
index 41174c95..995c817e 100644
--- a/docs/calendar.md
+++ b/docs/calendar.md
@@ -51,6 +51,8 @@ Calendars contain some of your most sensitive data; use products that implement
         - [:simple-appstore: App Store](https://apps.apple.com/app/apple-store/id1514709943)
         - [:octicons-browser-16: Web](https://calendar.proton.me)
 
+Unfortunately, as of January 2024 Proton has [still](https://discuss.privacyguides.net/t/proton-calendar-is-not-open-source-mobile/14656/8) not released the source code for their mobile Calendar app on Android or iOS. Proton Calendar's web client is open source.
+
 ## Criteria
 
 **Please note we are not affiliated with any of the projects we recommend.** In addition to [our standard criteria](about/criteria.md), we have developed a clear set of requirements to allow us to provide objective recommendations. We suggest you familiarize yourself with this list before choosing to use a project, and conduct your own research to ensure it's the right choice for you.