It's the 1st of December, so time to open that first window to start the 24 day countdown with an Advent calendar. Now, I know these calendars are most definitely a thing outside of Germany as well. But I can't recall them featured very prominently in much US media (that I consumed at least), so I kinda got the impression that they're not that popular over there?
Here at least, they are pretty much inescapable. Even if you don't buy one to put on your wall, pretty much everything will have one in some form; magazines and newspapers that have one as extra and/or to hide prizes to win in a competition, similar with daily TV or radio shows that draw a winner to "open" that day's "window", stores and shops both on- and offline use them to hide a daily offer, tons of websites that incorporate one in some kind of form (though that has lessened somewhat compared to the days when you could hide "awesome" stuff like screen-savers behind it), etc. But even aside from all these advertisement gimmicks, you may find some in Kindergartens, (early) school classes and even nowadays I'd have a hard time not to find at least one person bringing one in for work.
They can range from self-made ones, simple ones just with pictures behind it, some may have small toys or sweets. There are adult-oriented ones with alcohol or... well, also toys. Last year, they even had one made out of cargo containers on the Christmas market here.
By far the most common however (well, I don't have a statistic, but I'm relatively sure), are the ones filled with chocolate, pressed into forms that I swear is the same mold used by pretty much every company producing these calendars.