I believe I saw a SimGuru post an answer that prom is every week so if you have a sim joining high school at any time they have an opportunity to attend prom. I think this is great for those of us who play rotationally. I plan to have a different dance theme for each season like my high school had. If my sim at the time isn't interested then they won't go to the dance. I like that they decided to do this as gives everyone the opportunity to attend a prom. Would have been nice though to have made the prom an option on the calendar so player can determine if and when it is scheduled. Maybe the way the gameplay works limited that option? I don't know the mechanics of making things work in the games. Though we are able to schedule wedding events.