It's the 100th annual harvest festival in Yarrin and the night promises to have good food, fun, and drink under the bright harvest moon! What brings you as a traveler to the city? A chance to meet new people over a few ales? A chance to show off in the various competitions? Or just passing through?
Players should have their own reasons for coming to town and may (or may not) know the other PCs. I'll leave that to the discretion of the player. However, I'm not a huge fan of metagaming so if you don't have an introduction in game, it's likely your characters don't know each other.