With the coming of October, what's become an annual event has started back up to encourage contributions to Open Source projects: Hacktoberfest. This is a joint venture between Digital Ocean, GitHub, and Twilio. If you make the contributions, you'll receive a special limited edition Hacktoberfest t-shirt
[Here's the rules:] To get a shirt, you must make five pull requests (PRs) between October 1–31 in any timezone. PRs can be to any public repo on GitHub, not just the ones highlighted. The PR must contain commits you made yourself. PRs reported by maintainers as spam or that are automated will be marked as invalid and won't count towards the shirt. This year, the first 50,000 of you can earn a T-shirt (compared with 30,000 in 2017).
The guiding principles of the event are to encourage everyone to contribute (make them feel welcome) and to shoot for quality, not quantity. If you're not sure where to start, check out the main page for the event near the bottom of the page for a listing of projects with open Hacktoberfest issues.