The attendance of 52,762 was a regional record as well as a Guinness PRO12 one as rugby fans took advantage of cheap tickets.
Spectators, divided into East and West supporter zones, were even given the ability to vote on whether to have the Principality Stadium roof open or closed, with over 70 per cent choosing to keep the pitch covered.
On the field, Ospreys completed a bonus-point win over Cardiff Blues, their third in succession on Judgement Day, to secure their place in the Guinness PRO12 semi-finals.
With Wales head coach Warren Gatland watching on, Gareth Anscombe had the early upper hand on Welsh No.10 Dan Biggar as the fly-half kicked the Blues to a 9-7 lead.
But Ospreys built on their impressive opening team try by Dan Evans with two more tries early in the second half to soar into the lead.
First, a break from Alun Wyn Jones allowed Biggar to run in from 20 yards and Tom Grabham extended the lead when he went over the whitewash soon after.
The Blues' first try came through Josh Navidi but Ospreys had their bonus point when another excellent team move culminated in Ben John's score.
Replacement Dan Fish intercepted in the game's final play and ran the length of the field to add a consolation score for the Blues but Ospreys were comfortable winners as they went in search of a fifth Guinness PRO12 title.
In the second game Scarlets moved themselves back into the top six as they scored four tries to defeat the Dragons 29-10.
The Dragons led 10-3 early on after inside centre Jack Dixon carried strongly and offloaded to Tylor Morgan who raced over the line.
But two tries in the last five minutes of the first half put the Scarlets in firm control as Ken Owens crashed over from close range before a moment of magic from full-back Liam Williams.
The Welshman chipped over the on-rushing Dragons defence from inside his own half before collecting his own kick and giving Scott Williams an easy finish.
Liam Williams had a try of his own in the second half as he stepped through some weak Dragons tackles for his region's third try of the afternoon and wing Harry Robinson secured the bonus point with 20 minutes still to play.
With the derby double header continuing to grow and develop, WRU Chairman Gareth Davies, was full of praise for the dedicated fans at Principality Stadium.
"I would like to thank the Welsh public for flocking to the Millennium Stadium and helping to create what was a fantastic event," he said.
"The WRU and the four Regions have worked hard to create what was a great derby day and the supporters bought into and hopefully it can be a catalyst for the future.
"To set both a Regional record at the Millennium Stadium and a PRO12 record is testament to all those involved."
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