I think the fair, luau, and feast of the winter star make sense to be an all day event, as your farmer is milling about talking to everyone, playing games, and eating dinner (and everyone is theoretically opening gifts at the feast, we're just spared from having to sit through that as the player). The others could definitely get you home sooner. I would think 5pm would be reasonable. You get there between 12pm-2pm (at least no one I know ever leaves their farm any earlier) and spend a few hours at the event in-game. That still gives you enough time to hit the mines, fish, forage, or fulfill a request before everyone locks their doors, so you don't lose a whole day.
