While the 2021 NFL schedule has been finalized and every team knows the week they are playing each opponent, 10 teams don't know the exact date of the game they will play. This schedule anomaly occurs in Week 15 of the season, the first of three weeks the NFL will have Saturday games.
Five matchups have the potential to be "flexed" into the Saturday slate on December 18, which is Week 15 of the season. In Week 15, two of the five matchups currently designated as TBD will be played on Saturday with the remainder to be played on Sunday. Specific dates and start times for these designated Week 15 matchups will be determined and announced no later than four weeks prior to game day, so don't expect these matchups to be set until November.
The NFL is taking advantage of a Saturday window to showcase games with potential playoff implications or games that could involve a playoff race.