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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.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
About the calendar with CalDAV support, I always used the one that comes with my Zoho Mail free account and never had a problem with it.
To sync notes and todo.txt between devices, I use Syncthing, that is end to end encrypted and peer-to-peer, so there's no need to rely on a centalized cloud or self-host.