A good calender app for both Mac and IOS

iOS & iPadOS

Can any of you recommend a good calender app that works on both IOS and the Mac?
I've heard of Fantasticcal for IOS, but have read, that it isn't that accessible on the Mac.
I need an alternative calender since I have some events, that the buildin calender can't schedule. Like every 3rd month or half a year etc. This I can't get to work on the buildin calender on the iPhone or the Mac, so therefor I hope you have some great suggestions.
Thanks in advance