FOSDEM is an annual event taking place in Brussels, Belgium on a weekend (Saturday and Sunday) in January/February.
Besides general talks there are "devrooms" and stands resembling an expo. For both, projects need to register early in advance (December).
You need to set up and tear down your stand very quickly just before and after the event, so there is not much time and it's best if there are many people to help out.
Details for the events can be found here: