I'm guessing it was the first season since Big Bird had more of a dopey voice at the time. I heard it on MC Radio when he asked Susan and Gordon what it was.
While we're on the subject, I saw a sketch on You Tube the other day with Elmo singing it to Preston Rabbit. That was interesting, because I sort of thought Elmo's version of the song was just made for that CD. I never knew he really sang it on the show. I wonder how he could have known the song if he hadn't heard Big Bird sing it before.