Advent of Code is an Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like. People use them as a speed contest, interview prep, company training, university coursework, practice problems, or to challenge each other.
The first challenges unlock on December 1st at 00:00 EST/05:00 UTC, and continue every day throughout the month.
You can read more about the challenge here.