Ludumdare is the king of all jams! It has been going since 2002. The last one had almost 5000 entries.
It is held two times a year (usually spring and autumn) over the weekend starting Friday and ending on Sunday. In September there will be a theme suggestion and voting phase.
It would be cool to see you there, but don't feel obligated. My own participation will depend on how much work I'll have that week.