Advent of Code is an Advent calendar of small programming puzzles that can be solved in any language. It's been happening every December since 2015, and it's a lot of fun. Every day from 1st December to 25th December, a new puzzle with 2 parts gets unblocked. The goal is to solve all of them to save Christmas. The big story linking the problems together is great too. I've used AoC to learn some Elixir, it's a nice way to get some practice in a new language or to even try several ones!
Learn more and sign up for next edition at the official website.