For almost 100 years, the one thing the NFL didn't try to mess with was Christmas.
Until 1989, there were two Christmas games in NFL history, a couple of playoff matchups in 1971. There was criticism over those games intruding on the holiday. Even when the NFL started trying out some Christmas games many years later, it was rare. Through 2015, covering 96 seasons of the NFL, there were only 17 Christmas Day games. The NFL would move its normal Sunday schedule if it happened to fall on Christmas.
For different reasons, Wednesday was also off-limits. From 1948 through last season there had been only two Wednesday games in NFL history.