Peergos

Peergos is an encrypted personal cloud and the calendar is one part of it. Everything is end-to-end encrypted, your events included, and the server is open source so you can run the whole thing yourself and be the only one who can read it. Events are standard .ics files you can import, export, and share by link. It is broader and more experimental than a plain calendar app and the calendar lives in the web interface rather than a dedicated calendar app. It suits people who would rather own the server than rent trust.