It's the missionaries, Charlie Brown!

One of my favorite things about the Great Pumpkin is that it was in part inspired by missionaries from The Church of Jesus Christ of Latter-day Saints.

Charles Schulz's daughter Amy joined the Church and served as a missionary. He created several strips in which Linus' experiences going door to door parallel those of his daughter. 

And that's what the Great Pumpkin is all about, Charlie Brown!