Off the top of my head...
Sundance/Brianhead= Closed
PC/DV/Canyons= 14th
Brighton/Solitude= 21st
Alta= 28th
Snowbird- I think May 31st is their posted date, but if they get enough snow to justify it they'll stay open into June.
Closing day at PC is always a shit show, looking forward to it. Place was a complete ghost town today- emptier than I've ever seen it.