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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.
Proton recently updated their calendar to include an option to share a calendar with anyone (so they do not need a Proton account to see events).