Bath have brought forward their derby with Gloucester, which is scheduled for March next year.
The Aviva Premiership match, due to be played on Saturday, March 2, has been re-arranged for the evening before.
The match will now kick-off at 8.15pm on Friday, March 1.
Bath communications director David Flatman said the switch had been made because The Rec will be in use for the Bath Half Marathon on Sunday, March 3.
Flatman said: “Due to both the size of the Bath Half as an event and the fact the Rec is the event’s base, it would have been problematic to keep the game on the Saturday.
“A later kick-off has been chosen to give as many supporters as possible the opportunity to get to The Rec in time for kick-off on the Friday night – and with the advance notice we hope you can still plan to come to support the boys in one of the biggest games of the season.”
A 79th-minute try from Charlie Beech was of little consolation to Bath United as they lost 20-8 to London Welsh A at Imber Court on Monday night. Tom Heathcote provided the other three Bath points with a penalty.