NFL season used to end before Christmas. After the merger the first year that Christmas fell on Sunday was 1971. This was the divisional playoff round, and the Kansas City-Miami game was the longest game ever played.
This was unpopular and the NFL avoided Christmas there after. In 1976 Christmas was a Saturday, so they started the season a week early and Christmas weekend was the off week before the Super Bowl. In 1977, the divisional round was held on Saturday the 24th and Monday the 26th. The same was done for the wild card round in 1983 and 1988.
In 1989, with a new longer season, they played a single Monday Night Football game on Christmas night at 9 PM ET. The league has played a single game on some day related to Christmas (Eve, Eve of the Eve, or Christmas Day) in 82, 94, 95, 99, and 2000 and two games in 04, 05 06, single games in 09, 10, and 11, two games in 16 and 17, a single game in 20, and two games in 21. This year is the first where there are three games.
This is the last year of the TV deal. In the new deal, Fox will have the right to carry two day games on Christmas Eve or Christmas Day when it falls on Thursday, Friday, Saturday, Sunday, or Monday. The regular networks will have the rights to carry the night games when it falls on their night.