Kontact - KDE
Kontact: handle your emails, calendars and contacts within a single window
Calendar
fruux
Rainlendar
Rainlendar - Customizable desktop calendar
Calendar - Nextcloud
Lightning Calendar — Thunderbird
Organize your life — it’s about time!
Calendar Lock PEA
Etar-Calendar · GitHub
Android open source calendar. Contribute to Etar-Group/Etar-Calendar development by creating an account on GitHub.
Cron Calendar
Morgen.so
Fully private tasks and calendars in a single place. Connects to Google Calendar, iCloud, Exchange, Office 365, Zoho, Tutanota, Nextcloud, Gmx, Yahoo, and other CalDAV servers.