An announcement is made that an event registration has opened, an NPC in the Hub (or wherever) would appear (or open dialogue options on an NPC already there), asking players if they want to register for the event (with the hinkley requirement of having no gear on or in inventory to accept). If the player says yes, they are transported to a waiting room with all the other players that accepted, until a certain amount of time has passed and the "event" begins.
A few moments after registration is closed an announcement would tell the players in the waiting room what kind of event it was going to be, then shortly after this, the players in the waiting room would be moved into the event starting positions, whatever they may be for whatever event/map is randomly generated.
As for the events themself, there could be several different ones (different PvP scenarios, easter egg hunts, even just simple games of chance, etc), with some randomised variables (loot spawn locations etc), and the player would not know what event he was registering for (to stop people bringing their best alt for the particular events).