he 20th and final season of Law & Order premiered on NBC on September 25, 2009, for which it was moved from its previous time slot to air on Fridays at 8 p.m. ET for the NBC broadcast. L&O was moved to Monday nights on March 1, 2010 with a two-hour telecast at 9 p.m. ET, before settling into its new time slot at 10 p.m. the following week, March 8.

During the season, The Jay Leno Show in primetime was cancelled and Leno returned to the 11:35 pm Tonight Show because of affiliate dissatisfaction. Three more episodes of Law & Order were ordered to fill the primetime gap, increasing the number of episodes for the season to 23.

In early January 2010, NBC initially announced they were going to renew Law & Order a 21st season. NBC's President of Primetime Entertainment at the time, Angela Bromstad, said, "I’m a "Law & Order" junkie... I wouldn’t want to be responsible for not having "Law & Order" break the record." But on May 14, 2010, NBC abruptly cancelled Law & Order after 20 seasons on the air, tying it with Gunsmoke as American network television's longest-running, regularly scheduled primetime drama. The final episode aired on NBC May 24, 2010.

The late announcement of the cancellation resulted in Law & Order not having a "series finale" episode. Had the series continued, it would have been the last to feature S. Epatha Merkerson, who prior to the cancellation announcement had herself announced that she would not return for the 21st season.

Overview from themoviedb.org