It's happening always and never, ARTPOP is about possibilities
The event is happening in New York, so perhaps those living on the East Coast have the right time.
By the way, I did some googling and a Fashion Blog had this to say about the event a few days ago:
The first event, to present the Versus Versace J.W. Anderson Collection, is set for May 15 in New York and will feature a live performance by an as-yet-unnamed artist.
Wait... Donatella made it sound like Gaga randomly called her, but if they've been having meetings, then it's obviously been planned. Furthermore, you know Gaga knows we're expecting her to play new music. Maybe it's supposed to be a surprise?
The whole point of the break was to try to get people to forget BTW gaga (?) so it wouldn't make sense for her to reference her past work. Idk, hoping it's new music.