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 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.
The studio never intended to include the documentary about love in the film, even going so far as to tell Marc Webb not to film it. The audience reaction during the first test screening convinced them otherwise.
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 Goethe's 'The Sorrows of Young Werther'. Her line, when the idea for Tom to sing comes up, is: "I nominate young Werther here."
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.
Tom repeatedly associates the Patrick Swayze song "She's Like the Wind" with Summer. That song was originally recorded for the movie Dirty Dancing, 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, married Jennifer Grey in 2001.
One thing that both Tom and Summer have in common is their love for "Bananafish." This was the name of a band which gained national prominence for a brief period in the 80's, in turn named for the first of J.D. Salinger's "Nine Stories": "A Perfect Day for Bananafish". Actress Zooey Deschanel who portrays Summer is herself named for another Salinger story, "Franny & Zooey".
Red is intentionally not used and the colour blue is often conveyed to represent love or happiness (Tom dancing down the street after Summer first stays) Red does make an appearance, in Summers apartment you can see a little red origami bird. The only appearance is at the end when Tom meets Autumn. It is the colour of her blouse