If you use Linux, you might want to try Kalendar: https://claudiocambra.com/2022/02/12/kalendar-1-0-is-out/ (there is also some experimental work to have it work on Windows)
Disclaimer: I'm one of the core developer of Kalendar
Disclaimer: I'm one of the core developer of Kalendar
I wish there was a simple offline Calendar app that is isn’t bundled with email. Too bad Sunbird isn’t active anymore.