Uhm, I think you're confused... they've been on since day one.
Originally Posted by wikipedia
Debut Episode
The debut episode received mixed reviews and was considered to have "arrived needing plenty of work."[8] Critics noted that Jimmy Fallon was understandably nervous, but "remains nothing if not personable."[9] Interaction with the show's house band, The Roots was applauded and it was noted that "a bit in which Fallon sang a "slow jam" version of the news succeeded, in large part, thanks to Roots' typically taut playing and singer Tariq 'Black Thought' Trotter's impeccable voice and surprisingly good comic timing."[10] Critics commented that "the late-night role seems on the face of it a good fit" for Jimmy Fallon and that "this is a form that develops in the fullness of time, as chances are taken and limits tested and you learn the things you can learn only in the doing, night after night."[11]
Still doesn't help the show, it's terrible.
He seems so out of his element.