An edit-a-thon is an event in which people work together in person to build content on a wiki. Edit-a-thons can be very informal or very structured; they can be designed as primarily outreach events (to draw in new contributors) or they can be set up more to meet the needs of longtime contributors.
Further reading 
There are so many ways to plan and run an edit-a-thon, depending on the audience, the intended outcome, how much effort you want to put in, etc. Here are pages with more detailed outlines of how to do an edit-a-thon: