The Sunday prior to Easter. If you live in FL-- you get up real early and hack down as many palm trees as possible, slice them up, and give them to people at church. When mass starts, you wave them up and down, then play with them the rest of the time.
Mass is also really long because everyone in the church re-enacts Christ's passion. It's fun and all, especially when you yell "Crucify him" at the priest.
Palm Sunday is the first in a week long series of very looooong religious services.