Answer by taking a quote from the following article:

Tiaina Baul Seau Jr. (; SAY-ow; January 19, 1969 - May 2, 2012), better known as Junior Seau, was a linebacker in the National Football League (NFL). Known for his passionate play, he was a 10-time All-Pro, 12-time Pro Bowl selection, and named to the NFL 1990s All-Decade Team.

Seau announced his retirement at an emotional press conference on August 14, 2006. He called it his "graduation", because he was not going to stop working. He contended that he was merely moving on to the next phase of his life.  Seau returned to football just four days later, signing with the New England Patriots. He started 10 of the first 11 games for the Patriots, recording 69 tackles before breaking his right arm while making a tackle in a game against the Chicago Bears. He was placed on injured reserve on November 27.  On May 21, 2007, Seau re-signed with the New England Patriots for the 2007 season. In September 2007 Seau was named one of the Patriots' seven captains. He was a prominent contributor to the Patriots undefeated regular season that year. He started four of the 16 games he played in for the Patriots in 2007, and then started the Patriots' two playoff games before Super Bowl XLII against the New York Giants. New England's undefeated streak ended with a Super Bowl loss to the Giants.  After the Patriots had a number of injuries late in the 2008 season, they re-signed Seau. He started two of four games he played. On December 22, 2008, a fan was arrested for trespassing and assault and battery for tackling Seau as he stood on the New England sideline during a home game against the Arizona Cardinals. Seau stated that he did not feel threatened by the fan; he thought that the fan was happy and excited and got carried away.  On October 7, 2009, NFL Network reported that the New England Patriots had an "agreement in principle" with Seau for a fourth one-year deal; Seau took physicals and worked out with the team. He officially signed on October 13. He was active for 7 games for the Patriots in 2009, recording 14 tackles as a reserve linebacker.

when did he begin playing for the Patriots?
Seau returned to football just four days later, signing with the New England Patriots.