Well thanks to the Google "Is there anything we CAN'T do?" News Article Search Engine, you can read up on it.
From the creator himself in a St. Peterburg Times fluff article from Oct 23, 1966:
"It all came about," Shulz recalls, "when I was trying to write a sequence for the strip involving Linus' confusion between Halloween and Christmas...The article is an interesting read today as even in 1966 "It's The Great Pumpkin Charlie Brown" was being talked about as a "legendary" television special. Now 43 years later, I still enjoy watching it every year.
"... Linus is a youngster to whom everything must have significance; nothing is unimportant. Christmas is a big holiday and it has Santa Claus as one of its symbols. Halloween is also a special kind of day so it ought to have some sort of Santa Claus, too. That's what bothered Linus. And it bothered me. So, between us, we came up with The Great Pumpkin."
Read the whole article HERE.
If you're a time traveler and want to catch it, it will be "telecast" at 8:30pm on Channel 13 in St. Petersburg, FL on Oct 23, 1966.
Set your time circuits and be sure to have enough road to get up to 88...