But will Gareth Anscombe's conversion after Cardiff Blues' first try prove to be a post-match talking point?

Cardiff Blues outside-half Gareth Anscombe attempts a long-range penalty from just inside the Ospreys half but pulled it wide despite having the distance.

Blues are definitely looking on top as things stand.

Ospreys bounce back with a penalty from Sam Davies. Some confusion over whether earlier Gareth Anscombe conversion kick should have stood. But it counts at the moment!

Now it is the Cardiff Blues turn to throw the ball into touch when Matthew Morgan’s pass to Rhun Williams ends up on the sideline after a good tackle from Keelan Giles.

A breathless bit of play! Cardiff Blues centre Rey Lee-Lo breaks before Ospreys Fiji centre Josh Matavesi later intercepts.

He didn’t have the pace but also didn’t realise Keelan Giles was outside and then comically passes the ball into touch.

Illtud Dafydd
BBC Radio Cymru

Image source, BBC Sport

They've given out free flags on the seats but it doesn't look like anyone from the crowd has noticed

There are a mix of families, ardent regional fans in rugby shirts and some event goers in the stands.

Cardiff Blues draw the first blood as Kristian Dacey takes the ball from a line-out and does well to touch down in the corner of the pitch.

Gareth Anscombe takes the conversion that follows and, although it looks good after the kick, the ball hits the right-hand post and television replays show it may not have gone over but the extras are given.

Game on!

Dafydd Pritchard
BBC Sport Wales at the Principality Stadium

It's a bit strange having the roof closed with the sun shining outside - a bit like going to the cinema during the summer.

The main difference is that there are flame-throwers here. And 30 big blokes running into each other. Other than that, just like being in the cinema.

We are under way in Cardiff.

And out they come. The fire cannons are on and the atmosphere is building.

Kick off is moments away. Gareth Griffiths will be delivering live updates on the game from the Principality Stadium.

So then, who's going to come out on top today?

Arguably there's more resting on today's opener for Ospreys with Ulster and Scarlets breathing down their neck. But are they going to win or will Danny Wilson's Blues reign victorious today?

Let us know by tweeting @BBCScrumV., external

Image source, Huw

Of course it's not just regional pride at stake. There's still the matter of finishing places in the Pro12 that today's fixtures can help decide come the end of the season.

But just what can happen today and who's got the most to gain or lose? We went through the possibilities.

Image source, Huw Evans Picture Agency

Ospreys fly-half Dan Biggar has been ruled out of the Judgement Day Pro12 match with Cardiff Blues following his head injury against Leinster.

The Wales fly-half has not passed return to play protocols so Wales team-mate Sam Davies will start at 10.

Bradley Davies starts in the second row for the first time since November.

Ospreys: Dan Evans: Keelan Giles, Ashley Beck, Josh Matavesi, Dafydd Howells; Sam Davies, Rhys Webb (Capt); Nicky Smith, Scott Baldwin, Brian Mujati, Bradley Davies, Rory Thornton, James King, Justin Tipuric, Dan Baker.

Replacements: Scott Otten, Paul James, Rhodri Jones, Lloyd Ashley, Tyler Ardron, Tom Habberfield, Luke Price, Kieron Fonotia

Image source, Huw Evans Picture Agency

The Blues make four changes from the 24-24 draw with Ulster, with Sam Warburton, George Earle and Blaine Scully all injured.

Nick Williams, Macauley Cook and Matthew Morgan all start for the Cardiff region, while hooker Kristian Dacey returns from a groin strain.

Cardiff Blues: Matthew Morgan; Alex Cuthbert, Rey Lee-Lo, Willis Halaholo, Rhun Williams; Gareth Anscombe, Lloyd Williams; Gethin Jenkins (capt), Kristian Dacey, Taufa'ao Filise, Jarrad Hoeata, Macauley Cook, Josh Navidi, Ellis Jenkins, Nick Williams.  

Replacements: Matthew Rees, Rhys Gill, Anton Peikrishvili, James Down, Sion Bennett, Tomos Williams, Steve Shingler, Aled Summerhill.  

Dafydd Pritchard
BBC Sport Wales at the Principality Stadium

The atmosphere has been building nicely outside the Principality Stadium, with fans of all four regions enjoying the sunshine and the local bars.

There's an interesting vibe inside the ground too. As there are four teams playing as opposed to two, there are players and coaches dotted all over the place.

The Ospreys are here and, despite being injured, so is their usual captain Alun Wyn Jones.

The Wales lock will be his region's water carrier today. Service with a snarl.

Image source, BBC Sport

Image source, Getty Images

No pressure, guys. Four days until the British and Irish Lions squad announcement and what better way to stake your claim for a place in Warren Gatland's plans?

Robin McBryde says Judgement Day gives players a 'stage to be measured' for this summer's tour of New Zealand.

Who do you think could force their way into the side? Let us know by tweeting @BBCScrumV.