I don't think that A-Basin was ever open as late as September. Into July a few times and once into August.
FWIW, their average (median) closing date is 7 June. This year it was 4 June.
How about longest season? Probably, on average, K-Mart, Loveland, or Mammoth. (A-Basin didn't have snowmaking until recently so used to open around Thanksgiving.)
A-Basin was advertising in the spring of 1995 that with the huge base they had they were trying to stay open til Labor Day. I skied there on August 1, they closed August 9. With the now gone midstation loading on the Lenawee lift this made skiing til 4th of July fairly common in the 80's & 90's.
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Skiing is a disease curable only by death