Took a look around at the first date available for online tickets for some resorts that don't have a opening date published or announced on their website. Below are dates in November that I found one way or another for mountains that aren't owned by Alterra or Vail Resorts.
Killington has tickets on sale for Nov. 12. Won't be a surprise if Killington opens a lift for season pass holders on Friday, Nov. 11.
Pretty clear that Boyne Resorts wants all their resorts open by Thanksgiving if at all possible.
Nub's Nob in Michigan has a unique approach to the early season when it comes to online tickets. Advanced tickets are available now for Dec. 25 or later. Nub's goal is to open by Thanksgiving. They won't add online tickets until it's clear when the slopes will be open.
11/11: Ski Brule, MI
11/12: Killington, VT
11/14: Copper, CO
11/17: Loon, NH
11/18: Grand Targhee
11/18: Eldora, CO
11/18: Alta, UT
11/18: Sugarloaf, ME
11/18: Appalachian, NC
11/19: Lutsen, MN - weekends until 12/9
11/19: Purgatory, CO
11/24: Big Sky, MT
11/24: Telluride, CO
11/24: Taos, NM
11/24: Sunday River, ME
11/25: Sugar Bowl, CA
11/25: Jackson Hole, WY
11/25: Snowbasin, UT
11/25: Jay Peak, VT
11/25: Whiteface, NY
11/25: Gore, NY
11/25: Belleayre, NY
11/25: Jiminy Peak, MA - weekends until 12/16
11/26: Bretton Woods, NH
11/30: Snowbird, UT