It's the Grand Pumpkin, Milhouse is a parody of It's the Great Pumpkin, Charlie Brown. It was supposed to be called, It's the Great Pumpkin, Milhouse, but due to legal reasons, they had it renamed to the Grand Pumpkin.
Milhouse's prayer to the Grand Pumpkin is similar to the Nicene Creed.
All of the characters shown in the title sequence of "How to Get Ahead in Dead-Vertising" are: Marge, Patty, Selma, Edna, Grampa, Mona, Homer, Lurleen Lumpkin, Ruth Powers, Maggie, Rod, Ned, Maude, Todd, Medicine Woman (from The Simpsons Movie), Spiderpig, Lisa, Agnes Skinner, and Amber (from "Viva Ned Flanders") and Lindsay Neagle.
Nelson's line in his Halloween carol, "The Grand Pumpkin is super gay", caught the ire of anti-gay defamation groups who were protesting against people using the word "gay" to mean something inferior or bad.
When the celebrities in heaven are talking about the people telling about exploiting their images, George Washington and Abraham Lincoln are seen. It would be impossible for Homer to kill them, as they died in the 1700s and 1800s. Maybe Washington found his advertising portrayal offensive.
In the UK, the following scenes are cut from all three sections when aired on Channel 4:
"Untitled Robot Parody": The appearance of the Sex Toy Robot.
"How to Get Ahead in Dead-Vertisting": Krusty getting shredded in the woodchipper, Homer strangling, stabbing, and beating Prince with his own guitar, and Krusty shooting Homer in the head.
"It's The Grand Pumpkin, Milhouse": The Grand Pumpkin puking after learning that pumpkin bread is made of pumpkins (and not bread for pumpkins), The scene where Homer was carving Pumpkins and then being eaten after he was found by the Grand Pumpkin, Nelson calling the Grand Pumpkin a racist because he doesn't care that Nelson is going to carve a yellow pumpkin, The Grand Pumkin scooping the brains out of Skinner's Head and Tom Turkey blowing the Grand Pumpkin's brains out by blowing through a cornucopia (along with the Grand Pumpkin's dying words, although with the scene that was cut which involved Homer it might have been done so a plothole wouldn't have been made at all).
The Sex Toy Robot sounds like Andre (the sleazy guy Patty and Selma wanted Marge to date while Homer is in the hospital) from "Homer's Triple Bypass."
"Hungry, Hungry Homer": Homer sleeps on the roof of the house a la Snoopy from the Peanuts cartoons and Bart says, "Good grief."
"Treehouse of Horror XI": A "Treehouse of Horror" story where Homer dies and John Wayne is shown in the afterlife (only in Treehouse of Horror XI, he's in Hell).
"Saddlesore Galactica": Homer is chased by a group of angry people riding on horses (the jockey elves/the dead celebrities). Also, John Lennon's line about eating brains is similar to one of the jockey elves' threats to eat Homer's brain.
"Treehouse of Horror III": A "Treehouse of Horror" story featuring malevolent toys come to life ("Clown Without Pity"/"Untitled Robot Parody").
"Last Temptation of Krust" and "When You Dish Upon a Star": Reference to dead celebrities being used for advertisements.
"Treehouse of Horror II": Both episodes have homages to It's The Great Pumpkin, Charlie Brown.
"Lisa vs. Malibu Stacy": Lisa has a talking Stacy doll that's activated by a pull-string (even though pull-string toys are very outdated).
"Sideshow Bob Roberts": Homer votes during an election and a voting fraud scandal is revealed.
"Treehouse of Horror VI": The Gracie Films logo didn't have the organ or scream and instead was replaced with a "Philip Glass Homage" jingle of the third segment, whereas, in this episode, it uses the "Peanuts-style" jingle from the same segment, this is shown calmly in the episode.