Adjust your calendars! But don't get too excited.
Sunday Night Football and the NFL have taken advantage of flex scheduling for Week 13, moving the originally-scheduled Colts-Patriots match out of that slot and replacing it with a game between the Lions and the Saints. Because of various television contract rules, several games were moved around as a result, either between networks or in new timeslots. The Browns got a new timeslot. They are playing the Ravens in Cleveland that week, and instead of the game being at 1:00 PM EST, it will be at 4:05 PM EST.
One issue is that FOX is the network that has a doubleheader that week, so people outside of Cleveland in Ohio might not be shown the Browns game. Why? The Steelers and Bengals play at 1:00 PM EST.
By virtue of the Browns game not being flexed in Week 13, it is pretty safe to assume that the only chance the Browns have at appearing on SNF this season is if it takes place in Week 17 against the Steelers. The Browns already have a Thursday game in Week 14, the Week 15 game against Arizona isn't marketable, and the Week 16 game cannot be moved because of the Christmas holiday.