I don't know how it is that Linus Van Pelt can recite a passage from the Gospel of Luke (A Charlie Brown Christmas) and also believe in something like the Great Pumpkin, which I guess is a parody of Santa Claus after a sort.
However, that is the basis for 1966's It's The Great Pumpkin, Charlie Brown, which I believe is the 2nd Peanuts special, and one of the more enduring, as it'll return later this month.
The plot is rather simplistic. With the exception of Linus, who'd rather wait in a pumpkin patch, and is joined by wanna-be girlfriend Sally (Charlie's sister), the gang are out trick-or-treating. Lucy, appropriately, is wearing a witch's mask, given her crabby personality. There are the subplots we all know, like Lucy playing a football prank on Charlie Brown, and Snoopy engaging in a fantasy dogfight with the Red Baron. Poor Charlie still can't buy a break. All he gets in his treat bag are rocks.
Following is an excerpt with Linus & Sally in the pumpkin patch. Sally is so gullible as well as smitten.
ABC has broadcast rights now, and has for the last several years. It's also available on DVD, but more expensive than other Peanuts DVD's.