The 28-year-old lock forward arrived at Franklin's Gardens in the summer of 2008 from Stade Francais. In the three-and-a-half years since then Day has earned himself winners medals in both the European Challenge Cup and LV= Cup and played in the 2011 Heineken Cup final.
So far he has played 75 times in the black, green and gold and his form during his time in Northampton earned Day a call-up to the 2011 England Saxons Six Nations squad.
Day says that staying in Northampton was a straightforward choice to make.
"It was a fairly easy decision to make," he said. "The Saints are one of the top clubs in the country and my family are pretty settled here, so ultimately it was pretty easy to make the choice to stay in the area.
"The last four years have gone very quickly and over that time we have developed as a squad and have a good core of players which has grown in number. The Northampton fans have been treated to some good performances and now we're in a position where we should be competing for the top honours.
"The support we get from the fans is massive. Every player wants to be supported and feel like they're part of something when they turn up on a Saturday. A club like Northampton is good to play for; the whole town's behind us and I'm looking forward to being part of a squad that can hopefully deliver silverware in the future."
Director of rugby Jim Mallinder says that it is important for the Saints to be retaining the services of players like Day.