According to the DVD commentary, one of the film's writers estimates that 75% of the film actually happened to him.
During the flashback to Summer's (Zooey Deschanel) job selling ice cream, Tom (Joseph Gordon-Levitt) can be seen waiting in line.
Jenny Beckman, the girl who is mentioned at the beginning of the movie, was a real girl who dumped one of the screenwriters, Scott Neustadter. Summer is based on this girl, and the script of the movie on their relationship.
Around the time of the movie's release, director Marc Webb shot a short video for the Internet, which featured Zooey Deschanel as Sid Vicious and Joseph Gordon-Levitt as Nancy Spungen, in reference to a conversation in this movie.
Zooey Deschanel chose her own song for the karaoke scene, which was the Sugartown song.
The studio never intended to include the documentary about love in this film, even going so far as to tell director Marc Webb not to film it. The audience's reaction during the first test screening convinced them otherwise.
In the original screenplay, Summer was depicted as having cropped blonde hair to construct a more summer-like persona.
The screenplay for this film was featured in the 2006 Blacklist, a list of the "most liked" unmade scripts of the year.
The documentary about love was originally much longer and included opinions from Summer, Rachel, Millie, and Tom's mother.
During the office party, Summer and Tom get into a conversation about the validity of love. During the conversation, Summer makes a reference to Johann Wolfgang von Goethe's "The Sorrows of Young Werther." Her line, when the idea for Tom to sing comes up, is, "I nominate young Werther here."
This was the feature film debut of director Marc Webb, who mainly directed music videos.
Ian Reed Kesler, who gets in a fight with Tom (Joseph Gordon-Levitt) at the bar, is credited as an actual douche.
On the screens where the numbers of the days in the relationship are shown, the coloring and "mood" of the background art change to reflect the status of the relationship. Good days are brighter and bad days are darker.
The black-and-white movies Tom is watching at cinema are the parodies of the movies directed by Ingmar Bergman.
When Tom first enters Summer's apartment, the camera focuses on a bowler hat with a green apple on top of it. This resembles the famous painting "The Son of Man" by René Magritte, one of Summer's favorite artists.
The Smiths' "Louder Than Bombs," The Jesus & Mary Chain's "Psychocandy," and Public Image Ltd.'s "Album" can been seen in Tom's childhood room.
Joseph Gordon-Levitt wears T-shirts for Joy Division's "Love Will Tear Us Apart" and "Unknown Pleasures," and The Clash's "London Calling" albums.
The New Hampshire Greetings office and Tom's apartment were filmed in the same building.
Tom repeatedly associates the Patrick Swayze song "She's Like the Wind" with Summer. That song was originally recorded for the movie Dirty Dancing (1987), in which Jennifer Grey starred as the "she" that Swayze is singing about. Clark Gregg, who plays Tom's boss in 500 Days of Summer (2009), married Jennifer Grey in 2001.
This film is set in 2006-2007 (the narrator says at the end of the film that May 23 was a Wednesday. The last time May 23 was a Wednesday was 2007). However, Summer's e-mail message to Tom on Day 303 is dated May 7, 2008.
One thing that both Tom and Summer have in common is their love for "Bananafish." This was the name of a band that gained national prominence for a brief period in the 80s, in turn named for the first of J.D. Salinger's "Nine Stories: A Perfect Day for Bananafish." Actress Zooey Deschanel, who portrayed Summer in this film, is herself named for another Salinger story, "Franny & Zooey."
Zooey Deschanel and Joseph Gordon-Levitt had also appeared alongside each other in the movie Manic (2001).
The main character, Tom (played by Joseph Gordon-Levitt) says, "It sucks, why is it that pretty girls think they can treat people like crap and get away with it?" This line is a reference to Gordon-Levitt's previous role as Cameron in 10 things I hate about you when he says, "You know, just because you're beautiful doesn't mean you can treat people like they don't matter!"
The cover of Steven Patrick Morrissey's debut album, "Viva Hate," can be seen in both Tom and Summer's childhood rooms.
The Ringo Starr music album that Tom shows Summer in the vinyl store is the 1981 work "Stop and Smell the Roses."
The Adult film that Tom and Summer rent called "Sweet and Shower" is a fictional film and does not actually exist.
When Summer first comes around to Tom's apartment, the song playing is "Have I Been a Fool" by Jack Penate, which contains the lyrics "A kiss is just a kiss, I think that's what fools say."
Jean-Paul Vignon originally narrated the Swedish scene in French. In the final version, Jonas Åkerlund is narrating in Swedish.
This was the third time Joseph Gordon-Levitt had played a character named Tom. He previously played Tommy Solomon on 3rd Rock from the Sun (1996). And Tommy Burgess on Stop-Loss (2008)
In Summer's apartment, there is a copy of Paul Cezanne's famous "Blue Vase," which is an important part of Cezanne's influence to Cubism.
Both hometowns that the main characters are from, are fictional. The closest there is to a Margate, New Jersey is a Margate Township, but there is no actual city or town named Margate. There is no place in the state of Michigan named Shinnecock.
Patricia Belcher, who plays Millie, has a recurring role in Bones (2005) which stars Zooey Deschanel's sister Emily Deschanel.
The song being sung at the karaoke bar when Tom walks in is Every Rose Has It's Thorn by Poison.
The black Jack bottle of whisky is the same as Jack Daniels but with a different lable.