I don't think there's a calendar yet because they haven't been doing festivals for a full year so we can't be completely sure when and what the events will be and if they'll all be repeated every year.

There are a lot of holidays, festivals and celebrations in TES lore (although some are specific to certain regions or even certain towns and some probably aren't suited to in-game festivals, like days dedicated to specific gods or daedra). So in theory they could do a festival event pretty much any time.

Voxicity wrote: »

Next event is Midyear Mayhem - PvP event with double AP and achievements related to cyrodiil/BG's

I imagine it'll come late June/July

I haven't pvp'd yet, but I guess that would be a good time to do it