It's Rosenmontag, highlight of the Karneval, Fasching, or however it is called in the region. "Officially", the so called "5th season" already starts on November 11 (at 11:11am) and lasts until Ash Wednesday, but most people will only really notice in the last few days, when its taken to the streets. Originating as taking a last chance to act aloof and out of line before the fasting time before Easter... well, fasting time isn't really much of a thing anymore, so it's just an excuse to act aloof and out of line.
I guess I should put up a disclaimer: I'm not really much into it. A lot of the traditions, the (mostly political) satirical humor, the whole presentation in general can all be very... let's say, "special" and certainly not for everyone. For kids though, most of that doesn't really matter. Halloween may have been making its way over here in the last decade or two (though well beyond my time, so don't expect much on that regard here), I'm fairly sure that carnival is still THE time to dress up, so of course that's fun. And then you have the parades, where they throw "Kamelle" (i.e. sweets) into the crowd, which is even better if you manage to catch a lot. Though I should say, while the big parades, especially the ones in the "major" cites like Cologne, Düsseldorf and Mainz are happening on Rose Monday, many cities have separate parades, also some specifically for children, that usually happen earlier already. I figured I should still put it out today, since, well, as said, it's still the highlight day either way. For example, I remember that we went to school in costume in primary, though I guess the closer you get to the aforementioned cities, you'd even have the day off.