Scarlets and Cardiff Blues are to face each other for the second time in a fortnight by bringing forward their Pro14 match.
The league game will be played on Friday, 22 January in Llanelli - just under two weeks after Cardiff Blues defeated Scarlets 29-20 in the capital.
The match was scheduled for early March but will be at 20:00 GMT next Friday.
Wales' regions were facing two weeks off after the European Champions Cup and Challenge Cup were suspended.
The derby will give Wales squad players valuable game time before the Six Nations, as well as ensuring that Scarlets star Liam Williams is only unavailable for Wales' opening match against Ireland on 7 February after his red card in the last derby encounter.
Williams will also miss Scarlets' Pro14 game at home to Leinster on 30 January
European Professional Club Rugby (EPCR) says it had "no choice" but to postpone all matches scheduled for the weekends of 16 and 23 January after French government intervention.
This Scarlets and Cardiff Blues match is one of the final five rounds of league fixtures, though several other fixtures have been rescheduled after postponements because of Covid-19 or the weather.
These include Ospreys' trip to Galway to face Connacht, postponed in late November because of Covid-19, initially rescheduled for 13 February and now rearranged for 24 January.
This season's Pro14 competition has been shortened to 16 rounds to accommodate the new Rainbow Cup scheduled from April onwards- though still dependent on the virus crisis in South Africa and Europe.
The trip to Parc y Scarlets is the opening match for Dai Young's second term in charge at Cardiff Blues, after the interim director of rugby replaced head coach John Mulvihill.
It could also signal a return for Scarlets skipper and Wales hooker Ken Owens, who has been sidelined for three months with a shoulder injury, two weeks before the Six Nations opener against Ireland on 7 February.