According to A Charlie Brown Christmas: The Making Of A Tradition, Mendelson’s idea was to produce a documentary on the comic strip, and he met with creator Charles Schulz to get the ball rolling.
Peanuts and Charlie Brown exploded in popularity in the in the 1960’s, and television producer Lee Mendelson wanted to capitalize on it.
Mendelson eventually got a call from the McCann Erickson ad agency whose offer was not for a documentary, but a Christmas special.
The two came to an agreement for the documentary, and Mendelson went about shopping his project to advertisers.