All Proton apps need to be open source.

ALL of them.

I haven't yet paid for proton because ever since registering for an account, I just keep coming across more and more issues and paper cuts all over the place. It is concerning that I keep seeing people pointing out that the open source claims are not being fulfilled. Given the prevalence of blockchain encryption and p2p encryption/signing on networks like nostr, which are all open source and still manage to be encrypted, I wonder why Proton's offerings are still not.
We are in a global awakening of people realising that all these giant corporations are essentially evil. People are looking for new solutions. Open source is basically a necessity now. I can't start using Proton if it's not open source -- and ESPECIALLY not if they say "open source" but then aren't, or take yonks to actually open source their code.

Why they don't say anything about this? Are they planning on making it open source or no?
And their site says all their apps are open source which is incorrect.

I don't understand why the calendar isn't open-source yet. The app has been out for a while, and should have had all the time necessary to go through internal audits and security reviews necessary prior to open-sourcing it.

There is a link to github on your website, so it seems like you intend to open source it, the sooner the better IMO.

Having source code available under a free and open source license is one of the most essential features for a privacy and security focused service. Please make Proton Calendar's mobile apps open source. I'm not using this service until open source mobile clients are available.

They may not have all their design and layout features finalized, which can lead to chaos in terms of available development paths. I hope they get there this year though.

This! I feel like there are basic features missing that I would gladly help implementing, but that's hard to do with a closed source app...