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 interview prep, company training, university coursework, practice problems, a speed contest, or to challenge each other.
You can find more about it on the official website.
I didn't participate in the AoC challenge competitively this year but I exploited its puzzles to learn the Rust language.