I went to opening day at Vail once - that's a never-again! experience. "Madhouse" does not begin to describe the travesty that is Vail on opening day.
Round these parts, it's like bumpfreq explains - not very crowded, mostly ski patrol, some teeagers, and a couple of old farts like me and bf. Opening day is basically whenever they can manage to lay down enough manmade to cover the grass and they often only give an hour's notice or so - the website will say "opening today at 3pm" or whatever and those of us crazy enough to check the website every day head over to the hill.