I don't know if The Jim Henson Company has/ had any official documents as to when Kermit exactly debuted, but I have seen some Kermit toys that had a 1956 copyright on it's foot. I don't know if that means Kermit first appeared on Sam and Friends in 1956, or if the character (and possibly other Sam and Friends characters) wasn't copyrighted until a year later (it seems to be the case with many Sesame Street songs), or if the copyright only applies to the first national television appearance.
I think that both The Jim Henson Company and The Walt Disney Company officially consider 1955 to be the debut year. Disney celebrated Kermit's 50th anniversary in 2005, and an Ask Henson.com response pointed out that Kermit's birth year is 1955, also pointing out that when he made a guest apearance celebrating his birthday on Dinah! that he was a year off when revealing how old he is (I think Jim Henson might have known for sure when he first appeared, while the Jim Henson Company Archives may or may not have had any such documents, but then again, there was an interview where Jim Henson incorrectly confirmed that Kermit first became a frog and got his collar and flipers in The Frog Prince).