Advent of Code is a yearly event (from 2025 only 12 day's long, used to be 25). You get two coding challenges each day which you have to complete to get stars, you "win" by having all 50 stars by the end of the challenge.
My personal reason for joining this event is because I find it fun & challenging. It can also help to explore a new language by figuring out how to do these coding challenges in the language you are trying to learn, or see if you really understand your current language of choice by completing them with that one. My goal is to do it in GDScript first, and then move onto other languages such as Zig.