Fanboys

It's the Halloween party they've all been waiting for, to have fun with friends and hang out with fellow Star Wars fans and partners in crime. However, events take a turn when one of their bunch who long ago outgrew his old gang and took on adult responsibilities is suddenly in their midst again. Reconnecting is not easy. But soon, Eric, Windows, Hutch and Zoe realize that Linus will not live long enough to experience what's been on their minds for ages: The long-awaited theatrical release of Star Wars - Episode I, which is still several months away. Linus once again suggests to his friends what he's been scheming since fifth grade: Breaking into Skywalker Ranch, this time to steal a print of the movie. Crazy, right? But it might be the only way for Linus to see the movie before he dies. So as insane as it sounds, they take Hutch's van and embark on a mission to drive across half the country to the Ranch. -IndependentThoughtAlarmOn Halloween night, 1998, Eric Bottler (Sam Huntington) reunites with his old high school buddies Linus (Christopher Marquette), Hutch (Dan Fogler), Windows (Jay Baruchel), and Zoe (Kristen Bell) at a costume party. There is tension between Bottler and his old friends, due to Bottler being the only one that matured since high school. Bottler, now a successful car salesman, finds that his friends haven't changed a bit since high school; the number one thing they still have in common is their love of Star Wars. The gang expresses their anticipation for the latest installment to the franchise, Star Wars Episode I: The Phantom Menace. Linus proposes an idea that Bottler and he had been plotting since they were children: to infiltrate Skywalker Ranch and steal a rough cut of the film.
The next day, Hutch and Windows meet Bottler at work and inform him that Linus has cancer. The doctors estimate that he only has roughly four months to live; Episode I comes out in six. To make peace with his former best friend, Bottler decides to go through with their plan and infiltrate Skywalker Ranch. While on the road, Hutch decides to take a detour to Riverside, Iowa (the future birthplace of Captain James T. Kirk) in an attempt to start a fight with some Trekkies. Hutch gets his wish after attacking a Trekkie by the name of Admiral Seasholtz (Seth Rogen) in retaliation to Seasholtz calling Han Solo a bitch.
After arriving in Texas, the group encounters Harry Knowles (Ethan Suplee). After explaining their situation Harry gives them information on one of his contacts that knows how to successfully enter Skywalker Ranch. They are told to meet Harry's contact in Las Vegas, but before they get there they are arrested for fleeing a police vehicle. Zoe arrives to bail them out of jail, and accompanies them on their journey. Once in Vegas, Hutch and Windows make an attempt to have sex with some girls while Bottler and Linus go to meet Harry's contact. They are shocked to find that his contact is none other than William Shatner. Shatner gives them the information they need and leaves. Upon his departure, Seasholtz and his Trekkie friends, who were attending a Star Trek convention in Vegas, attack them. Meanwhile, Hutch and Windows discover that the girls they were with are prostitutes and their angry pimp (Seth Rogen) wants them to pay up.
The group escapes their adversaries and eventually arrive at Skywalker Ranch. Shortly after breaking in to the Ranch, they are discovered by security guards and are caught after a brief chase. The Head of Security (Danny R. McBride) tells them of their impending doom when he receives a phone call from George Lucas himself. Lucas tells him that he will drop all charges if they can prove to him that they are "fanboys." After a short quiz, the Head of Security confirms that they are fanboys and Lucas drops all charges. Being aware of Linus's illness, Lucas allows him to watch the film alone. After the film ends, Linus joins his friends around a campfire and mends his friendship with Bottler.
Six months later, Bottler, Windows, and Zoe emerge from their tent they used to camp out in while waiting in line for the first showing of Episode I. It is revealed that Bottler followed his and Linus's dream by becoming a comic book artist, Hutch has finally started his own detailing business, and Windows and Zoe are now in a relationship. Hutch arrives at the theater with beers he smuggled in, which they use to toast to Linus's memory. Just as the movie starts, Bottler asks his friends, "Hey you guys, what if the movie sucks?" [D-Man2010]

Question: On what occasion do the friends reunite?
Answer: A Halloween party