That Sunday was so good for Saints owed so much to Saturday.
Jim Mallinder’s men sat on, or maybe behind, the sofa as they watched the Champions Cup quarter-final line-up shape itself.
Exeter’s excellent win against Ospreys coupled with Bordeaux-Begles’ brilliant victory at Clermont Auvergne - Les Jaunards lost at home in Europe for the first time since 2008 - sent Saints through.
But though that was out of their hands, what happened on the previous evening was not.
Saints knew they needed to roll up their sleeves, grit their teeth and get stuck into a wounded Scarlets team.
And that they did, as well as showing plenty of ambition to get a try bonus point that eventually earned them a place in the last eight.
Had Saints not scored four times at the rain-soaked Parc y Scarlets, Ulster would have been the ones heading to Saracens in the quarter-finals.
But as it is, Mallinder’s courageous selection calls and Saints’ bravery in the Scarlets half paid off.
Yes, the opposition weren’t of the highest calibre.
Scarlets may be top of the Guinness Pro12, but they are currently ravaged by injury, missing so many star men.
That lengthy list of absentees did them no favours as they were routed 64-14 at Pool 3 winners Racing 92 on the previous weekend.
But the men in red were playing for pride.
They were determined to defend better.
And that Saints managed to break them down on four seperate occasions is laudable.
Mallinder’s men could have had more tries to their name, but they were happy to settle for four, and the five points they needed.
Then it was the waiting game.
The nail biting as Leicester Tigers let them down by suffering a heavy defeat at Stade Francais.
The palm sweating as Ospreys threatened to mount a comeback at Sandy Park.
But eventually it worked out.
And Saints will not fear going to Saracens, who they have beaten away in key matches before.
They will need a display akin to the one that saw them storm and conquer Allianz Park back in the 2013 Aviva Premiership semi-finals.
And wherever the game is played, they will have to be at their best and more to beat Europe’s current form side.
But a lot can happen between now and then, and the main thing is that Saints are still in the mix.
Every team who wins a major tournament gets out of jail at some point along the way.
Saints did that with Harry Mallinder’s late score against Glasgow Warriors last weekend.
And they walked out of the pool stage prison with that determined display at Scarlets on Saturday.
Now it is all on the day.
And Saints won’t mind that one bit.
How they rated...
Tricky conditions for the full-back, but he didn’t do anything wrong and made his tackles when he had to... 6
Showed plenty of desire to get in the game and his clever runs caused Scarlets a few problems... 6
An impressive showing from the Samoan, who formed a good combination with Harry Mallinder and grabbed a crucial try... 8
HARRY MALLINDER - CHRON STAR MAN
Two games, two tries, two man of the match awards; this Champions Cup business is easy, isn’t it? The young centre was in fine form on his second appearance in Europe... 8
A performance packed with desire as the former Scarlets wing scored on his old stomping ground. He was a constant menace.... 8
Didn’t manage to kick too well from the tee, but the conditions were tough and he was secure enough in open play... 6
Made his first Saints start and looked really sharp, making sure his side stayed on the front foot with some quick play... 7
The prop pushed his team on and his break led to George Pisi’s score. Also did well in the scrum... 8
A big display from the England captain elect as he poured everything he had into the game and showed why he is set to lead that Six Nations squad... 8
The young prop has fitted seamlessly into the team and he put in another assured showing on the big stage... 7
Took one for the team as Maselino Paulino landed a cheap shot on the lock, but it didn’t bother the Saints player who put in a towering display... 8
Such a vital presence for Saints as he helped them keep control of a game they simply had to win... 7
Only spent six minutes on the field after heading off for a head injury assessment and being unable to return
What an asset this man will be for Saints during the Six Nations. He has been superb since returning from injury... 8
Etched his name on the scoresheet in a big display as Saints got the tries they needed to progress... 8
Replacements (who played more than 20 minutes)
JON FISHER (for Gibson 6)
Was a good man to have on the bench and he was called for early on, proceeding to add his immense physicality to proceedings... 7