I used to think that myself......but realistically, it's an excuse we make.
Thanksgiving.....ok, 1.....maybe 2 days (including Black Friday leftovers) of indulgence.
Christmas.....same deal, Christmas Eve and Day could end up being 2 days of indulgence.
I actually maintained 16/8 fasting (started doing that consistently around Halloween or so) during both of those holidays.....although I didn't have any specific eating rules during the 8 hour window (i.e. reduced sugar, etc). Oddly enough, it was shortly after New Year's that I got off track a bit, but knew that Lent was starting earlier than usual this year, and that I'd have a relatively quick reason to get back on track.