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Online Calendar Software
(sopuli.xyz)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Proton has a nice UI and works well (I've used it for two years before moving to mailbox.org) but as of 6 months back, I was unable to make it work outside their walled garden. You can subscribe to proton calendar with any other calendar app, so you can read your calendar via caldav, but I found no way to edit it other than using their app.
I would recommand Proton for a lot of reasons, but if having a caldav calendar matrers to you I don't think Proton is a good option.