Halladay pitched the first 12 seasons of his career in Toronto, representing the Blue Jays at seven All-Star Games and winning the American League Cy Young Award in 2003 when he went 22-7 with a 3.25 ERA.
The 34-year-old will take the mound for the Phillies on Saturday in what will surely be an emotional day for fans and players alike.
"When I think about the great players that I played with here, Doc is right at the top of the list. Reflecting back on that, years from now, I think you're pretty proud that you were able to step on the field with a guy like that," said Blue Jays shortstop John McDonald, who played five seasons with Halladay. "Everybody in the city was sad when he left -- the players, staff, front office. ... He did some really great things here on and off the field."