For now I found calcurse (https://calcurse.org/), which also supports synchronization with Google Calendar. But I'm wondering what other text-based calendar apps you are using?
I have seen many different ways to achieve that in Emacs. Do you have one to recommend (modes, packages)? Do you synchronise calendars with other devices (e.g. smartphone, server)?
I don't actually use it myself as a calendar. I use org-mode for notes, todos and from time to time timing things I'm working on. But it has way more features than that right out of the box. But sadly, I can't really offer much in the way of advice for sync or anything like that.