The Deadly Apocalypse and Zombie Apocalypse events can be triggered at any time of the year by players. (Don't think we ever had a 100% lock on what the exact method was, but turning in costume salvage for an extra costume slot seemed to be part of it.)
I've seen each one about a couple of times each week.
That's not to say there isn't a chance that a GM or dev might be manually triggering events here and there to ramp up for this year's event, but seeing banners outside of the full-on holiday event period isn't that unusual.