Currently, the highly requested feature of integrating contact birthdays into the calendar seems delayed, likely due to the technical complexities of Zero-Access Encryption and syncing data between the Contacts and Calendar backends. However, a dedicated client-side solution could solve this issue efficiently for mobile users.
Since the Proton Mail app already decrypts our contacts locally on the device to display the address book, it has access to the birth dates in plain text. My proposal is to build an "Upcoming Birthdays" widget for Android/iOS that is directly tied to the Proton Mail app.

How it would work:
Instead of building a complex backend bridge between the Contacts and Calendar servers, this widget would strictly read the locally decrypted state of the Mail app's address book. It would simply scan for upcoming dates and display them directly on the homescreen.

Benefits:
No Backend Sync Needed: Bypasses the difficult server-to-server sync completely.
Privacy Intact: Requires zero server communication, maintaining full Zero-Access Encryption.
Huge Quality-of-Life Update: Gives users a practical and permanent way to keep track of important dates without having to duplicate every single birthday manually as a recurring event in Proton Calendar.

This client-side approach would be a fast, elegant, and highly appreciated solution for a core feature that many users are currently missing.