Context: Law & Order: Special Victims Unit, an American police procedural television series, premiered on September 20, 1999 on NBC. Created by Dick Wolf, it is the first spin-off of Law & Order and follows the detectives of a fictionalized version of the New York City Police Department's Special Victims Unit, which investigates sexually based offenses. SVU originally aired on Monday nights at 9pm / 8c EST, but it was moved to Friday nights at 10pm / 9c after the ninth episode. Showrunner Robert Palm felt too disturbed by the subject matter and left after the season's conclusion.

Question: when did the first episode of law and order svu air?
Steam of consciousness below:
To answer this question, we should know that: Law & Order: Special Victims Unit, an American police procedural television series, premiered on September 20, 1999 on NBC.
So, the answer is September 20 , 1999.