Advent of Code is a great idea of an advent calendar for programmers. I've been playing it every year for the last few years. I never tried to finish puzzles on the day they open or make quick solutions to get into the leaderboard (I'm also not in a very convenient timezone for that); instead, I typically learn a trick or three every year.
It's just a nice collection of X-Mas themed programming challenges, and if you're looking at this repo, you probably should try it some time :)