Green’s Dictionary of Slang

(US campus) a daytime party.

2008		Urban Dict. 17 Jan. 🌐 We're going to black out while it's light out at the darty!
2016		C. Eble UNC-CH Campus Sl. Spring 2016 3: DARTY — <day + party party during the day: ‘I don’t think I can go to the darty and a kegger later—that’s too much’.