Sunday's double-header at the Millennium Stadium is being see by Warren Gatland as the first trial before Wales tour South Africa.
The Blues will face the Scarlets while the Dragons take on the Ospreys at the Millennium Stadium this weekend.
Gatland plans to stage a Probables v Possibles match anyway next month before Wales embark, but has stated that this Sunday will be the first chance for players to put their hands up.
"Judgement Day is the first trial ahead of us picking the squad for the trip to South Africa," said Gatland.
"As far as we are concerned in the Wales management it's a chance for players to put a hand up and show why they should be in the squad.
"There's some fantastic duels in prospect with young pretenders like Dan Baker set to be up against Toby Faletau at number eight and a fit-again Matthew Rees locking horns with Ken Owens at hooker.
"We will be looking to take our strongest available squad to South Africa but eye-catching performances between now and then could be the difference between going and not.
"I'm really excited about Judgement Day because derbies tend to bring the best out of players. They're as close as you can get to an unofficial trial."
Gatland was speaking in Cardiff as he picked up his honorary doctorate from the University of South Wales.