First Day thumped home a header from Adam Rundle's corner just after the hour to equalise Lee Bradbury's opener for Oxford.
Then he got on the end of a free-kick from Stephen Dawson which was flicked on by substitute Richie Barker.
Lee Bradbury had bagged his fifth goal of the season to give Oxford a deserved 38th-minute lead.
The former Manchester City and Portsmouth striker has had to drop back into a wide midfield role to keep his place in the U's team.
But his class was evident as he started and finished the move, releasing Steve Basham on the left with a peach of a pass and then arriving in the middle to meet Basham's low cross. He controlled the cross with an instant first touch and shot low past keeper Kevin Pressman.
Before the break, Mansfield were restricted to mainly long-range efforts.
Chris Tardif comfortably saved an angled 20-yarder from Alan Russell and a scuffed effort from Nathan Arnold.
A speculative 35-yard shot from Gareth Jelleyman was more testing, but Tardif reached up high to save.
Oxford full-back Jude Stirling got ahead of Pressman at a corner, but couldn't keep his header down.
Oxford passed the ball well and had clearly gained confidence from their 3-0 win over Notts County on Boxing Day.
But they did not threaten Mansfield's goal enough, and once the Stags had equalised, they looked a totally different side, constantly probing for another goal.