Proton Calendar is a calendar service with a strong focus on privacy based in Switzerland. Proton Calendar uses zero-knowledge encryption, which means that the company can not read the content of your calendar even if they are stored on their servers.
European alternatives to Google Calendar
Google Calendar is a calendar service from the US-American company Google.
This pages lists European Google Calendar alternatives from companies based in a member state of the European Union (EU) or the European Free Trade Association (EFTA).
Tutanota Calendar is an end-to-end encrypted calendar service from Germany. They are available as an app for iOS and Android or through their Web app.
Calendar.online is a calendar service from Germany. They enable you to create a calendar and create different access links with different permissions for this calendar. For example, you can share a link with 'read without details' permission with someone you want to meet, and the person can see when you are busy without knowing why you are busy and reach out to you accordingly. You can also create sub-calendars to manage different kinds of dates.
fruux is a calendar service from Germany. Their CalDAV server is open source.
Use the chat in the right bottom corner