User blog comment:Dtuck010/Nightwing's Birthday/@comment-4803933-20120615025409

Upon the creation of the character, Dick Grayson's birthday was November 11 (ironically, Wally's new Earth 16 birthdate). After Crisis on Infinite Earths, the histories were all revamped and Dick's birthday changed to somewhere between October 24 and October 30. A third revamp after Zero Hour would give us Dick's current birthday of March 21.

In conclusion, we really don't know when Dick's birthday is gonna fall. The only date that's been ruled out, in my mind, is November 11, since they would've probably had a scene where Wally and Dick comment on their shared birthday. However, knowing this show and their tendency to cut out anything even remotely related to the Bat Fam, we'll probably have to wait until the final episode of the series airs and the final comic is released before we ever find out the answer to that question through Ask Greg.