The complete 2020 calendar
Obviously this is subject to change because it's Six Flags.
Opening Day is April 4th.
Right now the calendar actually implies no gap between Labor Day and Fright Fest on 9/11 but that seems like it could change because it's weird. I'm not saying it will, but it might.
Grape Adventure would (I guess) be November 7th and 8th which is the only non-HITP or FF weekend.
Holiday in the Park would start on 11/21 and end on 1/1 which is stupid since that's a Friday and they might as well run it until 1/3. I'd be surprised if they don't change that.