Just sort of...everybody. Like Town Drunk or Town Gossip or Prettiest Girl in Town. Have a few drinks in the local tavern and you'll find most everyone around has sort of come to an agreement on some things.
If you mean officially, though...depends on the place. In the bigger cities, you normally have some kind of festival committee appointed by city leadership to select the Satinalia King and Queen, plan the city-wide celebrations, all that sort of thing. Further out in the country I hear it's less formal, but I lived in Kirkwall my whole life. You'd have to ask one of the others how they do it out in the hinterlands. I think Curly's from the actual literal Hinterlands, actually. As in, that is the general name of the area.