It’s Hacktoberfest! Hacktoberfest runs each year in October to foster contribution towards open source projects. I’m in no way affiliated with the entities who run Hacktoberfest, but I like the idea of a focused time period when we are conscious of our contributions. It is also a great time for people that haven’t contributed to start!
From the three.js perspective, there isn’t that much that needs to be done. To get a bit more visibility, issues should be tagged with ‘Hacktoberfest’. PRs made to the three.js codebase count towards Hacktoberfest participation.
There are a bunch of documentation related issues: https://github.com/mrdoob/three.js/issues?q=is%3Aopen+is%3Aissue+label%3ADocumentation
As well as some bugs that might be good for someone getting started with the code base: https://github.com/mrdoob/three.js/issues?q=is%3Aopen+is%3Aissue+label%3A"Bug+(easy)"