Event Guide
General Event Guides
6_lasers - Reddit guide. Overview, shop info, and general advice.
Blue Archive Wiki - Link to the event information on the wiki.
Event Minigame Guides
6_lasers - Last year's minigame guide, the minigame remains unchanged.
General Tools
Event Planners
justin - In English. Updated whenever he feels like it, usually around the event date on global but sometimes is pretty late with it.
- /bag/ fork - A fork of Justin's planner made by a /bag/ anon, more features and actually up to date, you're likely better off using this instead.
I'll try, but no promises of keeping this up to date, bookmark the page or not, up to you.
If you're using the 
extension you can tell easily, if it's there it's updated if it's not there then it's not.