Desktop Bridge application
I do not keep my browser open all the time, and as I primarily use my laptop for on-the-go computing, I do not always have an Internet connection available. Even with a desktop computer on home WiFi or a smart phone, (rare) issues do sometimes occur with my carrier/Internet provider that prevent Internet access for short or extended periods.This makes integration with existing PIM (personal information management) applications crucial, both for access to event notifications even in those few rare moments where I do not have Internet access, and for the ability to sync up my calendars when I regain access to the Internet.
Ideally, a Proton Calendar Bridge application would support both CalDAV and WebDAV for maximum compatibility with PIM applications, though I would consider CalDAV to be the absolute minimum as most PIM applications will support this out-of-box.

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Alberto commented
Outlook Desktop integration is crucial
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David L. commented
There are quite a few different suggestions for this feature, expressed in different terms, but a lot of people would like Proton Calendar supported by a application for Windows or Linux instead of needing to rely on a browser.