Halo 3 Double XP weekend (which has already kicked off) and a new playlist for Fiesta (which spawns players with random weapons) called "Cinco de Mayo."
Two words of warning before you begin jamming cervezas and the right trigger: 1. Everyone can get double XP but you need the Heroic and Legendary map packs to play in "Cinco de Mayo." 2. The party only lasts until Tuesday, with the fiesta becomes a siesta.