Timing is everything with a project completion. Celebrate too early, still in the middle of wrap-up stress, and it feels like a false alarm. Celebrate too late and the energy is gone, with everyone already on to the next project.
The right moment: one to two weeks after the official handover or project closure. That's long enough to catch your breath, but short enough to keep the shared memory alive.
Account for diaries: plan early and communicate clearly. Don't make attendance mandatory, but don't make staying away attractive either. A good project completion is one everyone genuinely wants to be at.