Shared calendar apps

[Mummsnett]

I'd like to know what apps are best for those Co parenting to note who has kids when, unavailable weekends etc.
My ex is useless at remembering, so a shared calendar specifically which we use for all things parenting related would be useful.

we just send calendar invites via phone calendar. Works well for 6 years

We both have iPhones and you can categorise a calendar entry as ‘family’, we use this as it puts it in both of our calendars.