I have to say I don't know why, until some recent discussions on the #OpenBSD mailing lists, I have not heard of the built-in calendar(1) program, but I am totally sold on its simplicity. Really easy recurring and moving events and date calculations based on Easter etc. It is parsed by cpp(1) and so you can # include other calendar files in your personal one. I really really like this and will consider moving to this format -- 2-way sync with this via vdirsyncer or similar would be awesome to see.