I've never seen the Charlie Brown Christmas film before because I'm not American, and encountering for the first time it's a quite extraordinary artifact. My only exposure to it prior had been listening to the soundtrack album—which is the reason I wanted to watch it, and enough in itself to justify the whole endeavour, but the accompanying film is also remarkable.
The setup is that the depressive Charlie Brown is still depressed as Christmas is approaching, despite liking many of the trappings of the season. He's persuaded to direct a Christmas play, though all the actors seem more inclined to dance than act. He goes to get a Christmas tree for the play, and to strike a blow against commercialism, gets the only non-aluminium Christmas tree (apparently a real thing at the time), a bit of a weedy specimen, which everyone in the play proceeds to bully him remorselessly for. He bemoans that no-one can tell him what Christmas is all about, then Linus recites a passage from the Gospel according to Luke, Charlie takes heart, takes his tree home, tries to decorate it, fails and believes he's killed it, but his guilty friends come along and fix it up for him, and everyone sings Hark The Herald.
Describing it this way makes it sound like someone recounting a dream, the flow of events all coming a bit out of nowhere, but it doesn't exactly feel like that when you watch it. It flows from moment to moment in a way that reminds me of the animation style itself—somewhat jerky and discontinuous but having its own kind of elegance and making its own kind of sense.
I'm reminded of the thought I had when first reading Peanuts collections a few years ago: that Calvin and Hobbes wasn't so much "influenced" by this as just wholesale taking great chunks of it—except here it's not just another comic strip, it's for a whole attitude to reality. Hearing characters' depression and cruelty given voice by actual children is an excellent if somewhat uncanny choice.
It was apparently sponsored by Coca-Cola, which if done today would lead to the characters constantly winking to camera in reference. Here, you get a wholly straight-faced, resolutely (textually) uncommercial 25-minutes, where the climax is a lengthy verbatim Bible quotation. I love it, but it's exceedingly bizarre. I have no idea how it was framed on original airing but I'm imagining crashing from that into a Coke advert, and it's jarring in the extreme. Also, it apparently killed off the aluminium Christmas tree fad in America? I can abosolutely see why the network executives thought it would flop and why it has been broadcast every year since its first airing. It's only 25 minutes, I would strongly encourage you to take the time to watch it if you haven't before.