Most ski resorts open when there is at least 3 or 4 feet of snow, depending on the terrain. The exception would be those that have an all-year program of things to do there.
In my area (in and around N. Idaho) there are dozens and dozens of ski areas. In some years, they have not had enough snow until January or February of the next year. On the other hand, I have seen the surrounding ski resorts opening in October when there is a good, cold, early winter.
Of course, resorts at higher elevations may get colder and have snow sooner than the lower ones.
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