Specifically, it'll be on Tuesday, March 18th, at 8:00 pm (EST). That's when Marvel Studios will air a one-hour special on ABC, explaining its origins, going behind the scenes of itself, and discussing how awesome it is — none of which is at all important, except that it'll also feature the first look at the Avengers sequel.
Seriously, it's just going to be a hour-long back-patting session, but it will contain new footage from Captain America: The Winter Soldier and Guardians of the Galaxy, too, forcing us to sit through 55 minutes of self-congratulations to get to the good stuff. Of course, "first look" could mean anything — actual footage, some behind-the-scenes stuff, interviews with the cast where they talk about how great the movie is while never showing you any proof —but I'm afraid I'm still not willing to take the risk of not watching it, just in case. I'm not proud of this.
FYI, the awesome Avengers: Age of Ultron art above was made by Matt Broox.