The Celtics released their 2013 preseason schedule yesterday with the first practice game set for Monday Oct. 7 at TD Garden against the Toronto Raptors. But the biggest eye-openers are the two games scheduled against the Brooklyn Nets with Paul Pierce and Kevin Garnett lining up against their former team. The Celts will play at Barclays Center in Brooklyn on Tuesday Oct. 15 and then host Pierce and Garnett at TD Garden on Wednesday Oct. 23 in their preseason finale. The eight-game preseason lineup also includes contests against Atlantic Division foes New York and Philadelphia. The Celtics will play the Knicks at the Dunkin Donuts Center in Providence R.I. on Oct. 9 and will take on the Knicks on Oct. 12 at the Verizon Wireless Arena in Manchester N.H. The Celtics and Philadelphia 76ers travel to Newark Del. on Oct. 11 to play at the Bob Carpenter Center on the campus of the University of Delaware. The Celtics will also participate in the 2013 NBA Canada Series when they take on the Minnesota Timberwolves at the Bell Centre in Montreal Quebec on Oct. 20. All of the Celtics’ preseason games can be seen on Comcast Sportsnet New England. Tickets for Celtics preseason home games scheduled for the TD Garden will go on sale Thursday Sept. 5.