From The TV IV
The upfronts are conferences held in the spring where the U.S. networks announce news about their lineups for the fall. They promote the programs being added to their schedules, as well as announce properties returning for another year. The upfronts are designed to drum up interest in advertisers so they will purchase commercial time.
Cable networks also have upfronts, but they are scattered throughout the Winter and Spring. Comedy Central's is in mid February, Cartoon Network's is in early March, Adult Swim's is in late March, and TBS and TNT's are in April.