The sooner the better. Even in big years like 2001 and 2008 eastern lift service is hanging by a thread by the time you get to May. When I had a business trip East at end of April 1990 I went to Tuckerman's Saturday and Killington Sunday. It made me understand why there can be 2,000 people in Tuckerman's on a nice spring weekend.
If you want to keep skiing in May, come out here.
Season length: 21 months, Nov. 29, 2010 - July 2, 2012
Days in one year: 80 from Nov. 29, 2010 - Nov. 17, 2011
Season vertical: 1,323K in 2016-17
Season powder: 291K in 2011-12