2021 Flexible Scheduling Procedures
"Flexible scheduling" will be used in Weeks 11-18. Additionally, in Weeks 5-10, flexible scheduling may be used in no more than two weeks.
In Weeks 5-17, the schedule lists the games tentatively set for Sunday Night Football on NBC.
Only Sunday afternoon games are eligible to be moved to Sunday night, in which case the tentatively scheduled Sunday night game would be moved to an afternoon start time.
For Week 18, two Saturday games and the Sunday night game will be announced no later than six days prior to January 9. The schedule does not list Saturday games or a Sunday night game in Week 18, but games with playoff implications will be moved to those time slots.
Flexible scheduling ensures quality matchups in all Sunday time slots in those weeks and gives "surprise" teams a chance to play their way into primetime.
https://www.nfl.com/schedules/flexible-scheduling-procedures