There's an federated calendar/events site in development called at @calendar_social

It's still very early days, but if you're interested or want to help out, have a look:

The developer is @gergely

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Mastodon is open to all users and federates with most instances.

🇩🇪 🇦🇹 🇨🇭 ist offen für alle User und ist mit vielen anderen Instanzen verbunden.