Thompson and Grayson helped Saints to claim Heineken Cup glory in 2000, while Day was a key figure in the club's Premiership and Challenge Cup triumph of 2014.
Grayson – already a member of the Hall of Fame at Franklin's Gardens – is Saints' all-time points scorer, having racked up a huge 2,786 points in an 11-year career in black, green and gold that spanned domestic finals, the club's first-ever Heineken Cup win and culminated in lifting the Rugby World Cup trophy with England in 2003.
And after hanging up his boots in 2005, the British & Irish Lion turned his attentions to coaching and remained a part of the set-up in Northampton until 2012, guiding the side through promotion, Challenge Cup and Anglo-Welsh Cup titles and to a Heineken Cup final in 2011.
Meanwhile, Thompson donned Saints colours some 192 times after graduating from the Academy to make his club debut in 1997.
The former club captain featured alongside Grayson in the momentous Heineken Cup win and would also lift the Webb Ellis Cup alongside the fly-half in 2003, before embarking on a British and Irish Lions tour in 2005 ahead of his retirement from the game in 2011.
And the pair are joined in this year’s class of inductees by fellow Saints legend Day, whose 10-year career in Northampton coincided with one of the most successful periods for the club.
The lock made 226 appearances in black, green and gold and helped Saints to their first-ever Premiership title, as well as two Challenge Cup trophies and a Premiership Cup victory.