I don't know if this is 100%... I'm not a skiier, so I assume it depends on
But ideally, yes, the slopes are open in May. I heard something about a snowboard event being held there in mid-May. I wouldn't normally know this stuff, but I am going on a vacation next month myself and our original destination was BC.
I think their site is http://www.whistler.com or http://www.whistler-blackcomb.com. I'm sorry I don't have web access right now to check for sure, but I do know they have a good site, so if these don't work, do a search and it'll come up.