A bugday is an event where the QA team meets contributors to work on bugs (triage them, investigate them, verify them, update them). The QA team organizes two such events every week on the IRC #qa channel:
If there are no events one week, or any day you want to contribute, feel free to access the wikis for these events and work on the bug lists there. Contribution is welcomed at any time! When in need of help, you should be able to find it on the IRC #qa channel.
See also QA/Community/Bug Day.