Ken Pisi had already scored two tries for Saints but the game looked to be petering out at 21-6 until academy scrum-half Day came off the bench. The England Under-20 youngster scored his first try after 72 minutes and dived over for another another six minutes later.
Both sides gave their fringe players and youngsters a run out, although Gloucester had Freddie Burns captaining the side, with Mike Tindall and Charlie Sharples marshalling the young backs.
Kiwi fly-half Glenn Dickson, making his debut for Northampton alongside fellow New Zealander Rob Verbakel and academy wing Howard Packman, gave the home side a 3- 0 lead after five minutes thanks to a chargedown by second row James Craig causing chaos.
Three minutes later Burns levelled the score and then punted the visitors 6-3 in front, before a scrum penalty saw Dickson, a summer signing from Otago, make it all square again.
Gloucester's decent start was undone when Cox was sin-binned in the 22nd minute for sticking a leg out as Saints scrum-half Ryan Glynn ran past.
A minute later Northampton had the first try of the game when Dom Waldouck pierced through the Gloucester defence, chipped a superb grubber with the outside of his boot to the right wing for James Wilson to kick ahead and Pisi to dive on the ball.
Pisi grabbed his second five minutes later when Glynn fed him 40 metres out down the short side off a lineout drive and the Samoan skipped over a tackle before cutting inside to score. Dickson converted to make it 18-6.
Alex Waller, making his 100th appearance in the front row for Saints, ensured the home side dominated the scrum with a man-of-the-match display at loosehead.
Gloucester brought on the more experienced Yann Thomas and Koree Britton in the front row but it made no difference as Northampton began the second half with Dickson kicking three points from yet another scrum penalty.
Gloucester tried to up the pace and although Cox went close it was a disjointed affair, with plenty of stoppages for injuries, the worst looking like a broken arm for Northampton lock Craig.
It had been a drab second period but a trio of Northampton replacements brightened things up as fly-half Sam Olver flicked a clever inside ball to Matt Williams who off-loaded for Day to score his first try for the club. And with two minutes left Day was on the spot as the ball was spat out of a scrum to dive over for the bonus point.
- Sports & Recreation
- Glenn Dickson