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The Global Game Jam (GGJ) is the world's largest game jam event. A project of the International Game Developers Association (IGDA), it occurs annually in late January, and brings together thousands of game enthusiasts, participating in hundreds of local jams around the world. Attendance at each site might be anywhere from 1-300, though is typically 5-25 people, usually students, though many professional game developers and other interested hobbyists attend as well.
The structure of a jam is usually that everyone gathers on Friday afternoon, watches a short video keynote with advice from leading game developers, and then a secret theme is announced. All sites worldwide are then challenged to make games based on that same theme, with games to be completed by Sunday afternoon.