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.
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.
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."
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.
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.
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."
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.
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!"
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.
The trivia items below may give away important plot points.
Red is intentionally not used in this film, and the color blue is often conveyed to represent love or happiness (Tom dancing down the street after Summer first stays, etc.). The color red does make an appearance, in Summer's apartment where a little red origami bird can be seen. It makes a final appearance at the end when Tom meets Autumn, and the color of her blouse is red.
There is a fan theory that the final conversation between Tom and Summer on the park bench was all imagined by Tom and did not really happen. If that is accurate, then Day 408 (at Summer's house party when he learns of her engagement) is the last time Tom ever sees Summer.