The Rays claimed Day off waivers from the Red Sox before they signed right-hander Lance Cormier on Friday, which created a need for a spot on the 40-man roster. Day was designated for assignment to make room for Cormier.
Day, 28, still has an invitation to Major League camp for Spring Training.
Day spent the 2008 season in the White Sox's farm system, spliting time between Double-A Birmingham, where he went 1-6 with a 6.61 ERA in 13 starts, and Triple-A Charlotte, where he went 0-3 with a 4.56 ERA in 21 relief appearances.
The Red Sox claimed Day off waivers in October.
Day pitched in 13 Major League games for the White Sox in 2007, posting an 0-1 record with an 11.25 ERA in 13 games.
Day, who stands 6-foot-4, 210 pounds, will compete for a job in the Rays' bullpen in Spring Training.
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.