Not tied to a specific day, but going on through most of December (if not longer): Christmas markets. And they are not just in major cities, even small ones like my hometown has one. And in case of big cities, some of its districts may have small markets of their own in addition to the main one.
While it may seem like a Christmas-themed fair at first glance, that's not really fitting. With a fair, I would associate rides and spectacle and while you may find a few carousels for the smaller kids, the focus is certainly more to have a tranquil... shopping trip. I mean, it is a market after all. You have many, many little stalls, often in form of decorated, cozy wooden huts, selling Christmas ornaments and decorations, toys (wooden ones, teddy bears or dolls, plastics and electronics wouldn't really fit), clothes (though again, mostly stuff fitting the theme, ie. knitted sweaters, hats, scarves or gloves) - and lots, and lots of food. And in contrast to a regular market, the focus is less on things for your everyday life, and more on stuff to consume then and there. Sure, there might be someone selling bread, smoked fish, spices, but that's maybe one or two stalls for each. But you'll have plenty to chose from for sweets (of course), baked potatoes, roasted almonds, waffles, crêpe, meat in various forms, but especially as Bratwurst and variations of it.
And on the liquid side of things, there's Glühwein. These are usually the biggest stalls, several of them all over - and they are still the most crowded. While that's certainly not for me, it's actually the reason for many adults to go there (the term "Glühweinbetankung" (basically "fueling up" on it) as colloquialism for "going to the Christmas market" is a thing) and even if not, most still get one.
I rather stick to a Bratwurst though. ^^;
* not pictured: The actual mass of people visiting these. ;P