Excerpt taken from wikipedia- "Toonami was Cartoon Network's primary action-animation block. The block, which made its world premiere on Monday, March 17, 1997, initially replaced Power Zone, Cartoon Network's most recent incarnation of the Super Adventures block which had been a staple on the network since October 1, 1992. Toonami was originally a weekday afternoon cartoon and anime block hosted by Space Ghost villain-turned-producer Moltar at the Ghost Planet Industries building from 1997 to July 9, 1999.
On Saturday, July 10, 1999, Cartoon Network relaunched Toonami with a new environment, the Ghost Planet Spaceship Absolution, and a new host named TOM. The night also introduced the Toonami Midnight Run late night block which was originally a five-hour Saturday night block (technically Sunday) at midnight EST until March 2000, when it moved to weeknights in an hour-long format until January 2003.
On Saturday, April 17, 2004, Toonami was moved from weekday afternoons to a Saturday evening slot, where it aired for four hours starting at 7/6c. Beginning October 27, 2007, it aired for 2 Hours starting at 9/8c. On September 20, 2008 Toonami ended, with its final broadcast that same day"