Municipal Christmas decorations often stay up far beyond that, attesting not to any tradition but only to the sluggishness or negligence of municipal services.
That's a little unfair. I'm glad our town's beautiful decorations stay up until Candlemas. This winter has been extremely gloomy so far, and the lights cheer up the place no end. This year they are projecting moving snowflake patterns on the ratusz, and it looks great.
A very old tradition with pre-Christian origins, aka the Feast of Fools, when the usual social order was overturned (see Shakespeare's eponymous play). At the old universities the dons would serve the students dinner.