A One day stand is a more innocent form of "one night stands," It's when you meet someone for the first time, flirt with them constantly all day, call eachother cute/pretty, hold hands, and maybe even kiss. Then you never talk to them again after that.
Person 1: "Are Julie and Zack going out?"
Person 2: "No, they kinda just had a One day stand."
Person 1: "ohhh,sucks."