“If you’d like to take part in this frivolous event, travel to the pavilions located outside the cities of Ebonheart, Vulkhel Guard, and Daggerfall. Once you find a pavilion, talk to one of the three Jesters convincingly dressed as Jorunn the Skald-King, Queen Ayrenn, or High King Emeric to get started. Complete the intro quest titled ‘The Jester’s Festival’ to acquire the delicacy known as the Pie of Misrule. (If you completed the quest last year, you won’t need to do so again.) When you use this mouthwatering Memento during the event, you’ll receive a two-hour 100% XP buff. You can continue to enjoy the pie once the event is over, but you will no longer receive the bonus.”
And yes, if you’re not into eating pie, you can throw it at other players and NPCs for rewards. Protip, this works only in Tamriel, not real life.