|Holidays of Future Passed||
- The title is reference to the Moody Blues album Days of Future Passed or the X-Men storyline Days of Future Past.
- Krusty's televised rant and his surroundings are similar to those by the late 60 Minutes personality, Andy Rooney.
- The number on the door of Barts rented room at school '203' could be a reference to the freedom writers.
- When Lisa first enters the Ultranet, the first thing she does is respond to friend requests, most likely those from Facebook.
- Several of Homer's ships in bottles are fictional futuristic spaceships such as the Discovery from 2001: A Space Odyssey and the Planet Express ship from Futurama.
- The air travel shown is highly similar to themes and characters of the Mad Max series.
- When Marge makes the cookie recipe appear may be a reference to Back to the Future: Part II.
- Me and Julio Down by the Schoolyard plays when Homer takes Bart's kids to find his dad.
- "Sleigh Ride" by She and Him plays during the montage of Simpsons christmas card photos through the years.
- One of the teleporters goes to New New York from Futurama.
- In Zia's Usernet room, Empress Zimzam is a resemblance of Padmé Amidala from the Star Wars prequel trilogy, with her makeup and dress also having a resemblance of Padmé from The Phantom Menace. Her name may be a reference to the real world empress, Wu Zetian.
- London is seen to be patrolled by bobby-wearing Daleks, an alien race from Doctor Who , along with the titular plants from Day of the Triffids .
- The Benny Hilton is a play on comedian Benny Hill whose iconic chase gag is reproduced by the Dalek to the sound of Yakety Sax.
- When Lisa uses the Google motor research, the background seems like the doors room in the Pixar film Monster Inc.
- Dr. Hibbert is seen wearing a VISOR like the one worn by Geordi LaForge in Star Trek: The Next Generation.
- The tube Abe is in is a reference to the tube that appeared in the very first Futurama episode.
- Maggie's performing style as a singer may be inspired by Lita Ford and/or Lady Gaga.
- Lisa married Milhouse but seems to have keep her maiden name, since her daughter's full name is Zia Simpson. Lisa's Christmas card is captioned "Merry Christmas from Simpson-Van Houten-Simpson-Simpsons" which may imply that Lisa kept her last name, Milhouse took on the hyphenated Simpson-Van Houten surname and Zia has her mother's last name.
- Martin Prince has undergone a sex change to become Martha Princess.
- Just like previous flash-forward episodes, Maggie never says anything. The robot nurse told her not to say a word for the duration of her pregnancy from that point forward, due to new research discovering the the umbilical cord is also a vocal cord. After she has her baby, she does open her mouth to speak when Grandpa asks who her baby's father is, but he cuts her off. However, she is shown to be a world famous singer, enhancing the gag through irony. Similar to Lisa's Wedding, where it's revealed she is as a good singer, but gets interrupted before she can use her talent, along with being cutoff whenever she tries to speak, and being described as never shutting up.
- On Maggie's Christmas Card, it is captioned "Maggie Simpson: The Voice of Her Generation".
- When Homer visits his dad in the cryogenics facility, other people seen in suspended animation include, Crazy Cat Lady, Superintendent Chalmers, and Agnes Skinner, Edna Krabappel, Old Jewish Man, Hans Moleman, Professor Huntington, and Jeffery Albertson. The last time an elderly character was frozen was in Lisa the Simpson where Jasper freezes himself in the Kwik-E-Mart. This time Abe actually became unfrozen in the future while Jasper thought he was in the future when he became unfrozen.
- Lenny and Carl have switched brains.
- This is the fourth episode set in the future and longest gap between two episodes set in the future.
- According to Ned, Homer is the reason Edna is dead, meaning they were together well into the 30 years between settings. This is also a play on Homer being the reason for Maude's death.
- Nelson and Lisa's relationship from Lisa's Date with Density is referenced as Nelson tries to win back Lisa.
- Lisa also asks Bart if she should have married Nelson, indicating that they were once together. This also indicates that Nelson proposed to Lisa sometime before she married Milhouse, most likely at the same time as Milhouse had proposed. Later in the episode, when Lisa is going through her friend requests in the Ultranet, Nelson's request appears and he asks her is her marriage is still dead and unfulfilled. Lisa probably described her marriage like this to Nelson in her phone calls to him. She rejects his friend request, however.
- Jenda from Future-Drama appears in a picture. It's revealed that she married Bart, had two children with divorced him, and married her new boyfriend without Bart knowing.
- At some point, paper cash will no longer be in use, according to Apu.
- Schools at this time period are now banned from talking about the meaning of Christmas, according to Bart's sons.
- Chalk outlines of Sideshow Bob and the Capital City Goofball can be seen on the floor at Moe's Tavern.
- The name of the father to Maggie's daughter is never mentioned.
- However, in this episode's sequel, "Days of Future Future", Maggie makes a cameo at the Guilded Truffle, where she is sharing a drink with Gerald Samson, meaning there is a good chance that he is the father.
- When Lisa dates girls in college, she has shorter hair, probably due to the stereotype that lesbians have short hair.
- We learn Sanjay Nahasapeemapetilon was killed in a hit and run, based on what Apu explained.
- Mr. Burns last released the hounds at Christmas in Dude, Where's My Ranch?
- Originally, this episode was supposed to be the series finale, as Fox was considering cancelling the show due to money issues stemming from advertising the show. Once the issues were resolved with a pay cut for all involved with the show, the series was greenlit for seasons 24 and 25.
- Among the people, who sent requests from Springfield Elementary School to Lisa was Lewis (?), Groundskeeper Willie, Miss Hoover, Janey Powell, Sherri and Terri, Allison Taylor, Dewey Largo, Edna Krabappel (see "Goofs", why might this happened), Martha Princess, Nelson Muntz,and Wendell Borton.
- If Zia is 13 years old and this episode is taking place in 2041, then this places Zia's birthdate somewhere around 2027 or 2028 depending on what day Zia was born.
- Lisa married Milhouse when she was 23, the same age she got engaged to Hugh Parkfieldin "Lisa's Wedding.
- In the four years later picture, Lisa is shown to be a Goth. Her goth design is the same from Smart and Smarter when Lisa was trying to change her image.
- Maggie looked exactly like Lisa growing up in the opening up until she is 16.
- Maggie's hairstyle as an adult and teenager is similar to her hairstyle in "Lisa's Wedding". However, as an adult Maggie has 7 "points" of hair instead of her usual 8. However, throughout most of the episode, Maggie's hair is disheveled and is in 9 "points". It is in 7 when she is at the hotel and returns to this after she is checked into the hospital.
- Lisa's hairstyle as an adult and teenage is similar to her hairstyle in "Bart to the Future".
- This is the last episode to air in 2011.
- This episode could've served as a series finale for the show.
- In the description of the Sky 1 airing of the episode, there is a mistake saying it is the fifth episode of Season 23, not the ninth.
- There are only 42 octuplets of the octuplets. If all of the octuplets had octuplets, there should be 64. This is either a goof or not all of them are there.
- Grandpa's hat is slightly lower in the reflection of himself.
- Carl (with Lenny's brain) tells Lenny (with Carl's brain) to stop making Lenny fat, but in Sweets and Sour Marge, Lenny is one of the people in the sugar, and even looses some of his teeth.
- When Lisa enters the Ultranet she comments that it is Dr Seuss' birthday as indicated by the design of the Google page. Dr Seuss' birthday is March 2 and this episode takes place over Christmas.
- When Lisa first goes into the Ultranet there are two of the same people in one scene.
- Despite Edna being killed by Homer, she appears at Lisa's Ultranet friend requests. However, it could be possible she sent the request before she died. This gives off the impression that Lisa isn't a very frequent Ultranet user.
- Zia's drool is missing when we see her and her mother are in the table while both are on the Ultranet.
- Lisa and Homer refer to Prince Harry as one of history's tyrants, strongly implying that he eventually became King, though they still refer to him as a Prince rather than King.
- The family is grown up in this episode but remain the same voices.
- This also happened in Barthood.