To most people, the Boston Marathon is a race hosted annually by the City of Boston on Patriots’ Day in April. But to a select band of individuals from across the country, “the” Boston Marathon happens in February, and requires 24 hours, not 26 miles, of endurance. Every year since 1976, a few hundred fans have spent one full day enjoying the best and worst of the last century’s science fiction cinema.

The festival’s location has changed since the event first began—moving from the old Orson Welles Theatre in Cambridge (which burned down in the mid-80s) to West Newton to its present Davis Square home. And each year’s event has a different theme (this year’s was “Alien Attack”). But the close-knit audience remains more or less the same; there are multiple people who have been to all 34 festivals.

For a stranger to the sci-fi community, the overnight festival—which took place this year from last Sunday to Monday—can appear somewhat inscrutable. Little seems to link the bizarre assortment of films, ranging from the most camp B movies of the 1950s (“I Married a Monster from Outer Space”) to new releases (“Chrysalis”) to, perhaps strangest of all, mass-marketed Hollywood fare (“Transformers”). But fans point to the distinctive worldview that sci-fi films provide as a unifying thread through the festival.

“I like the positive view of the future,” says Jim Peterson, a 10-year attendee of the event. “Science fiction shows us new ideas.”

“It’s meant to be escapist,” says Jen Simon of Philadelphia. “It’s less mundane than normal life.” Simon is on her second trip to the festival; she was introduced to it by her fiancé, Steve Lento, who’s been a regular at the festival since middle school.

The highlight of this year’s festival was the recently-released “Alien Trespass.” Directed by former “X-Files” producer R.W. Goodwin, the film pays homage to five iconic 1950s sci-fi films. Any great sci-fi movie, says Lento, “should be aware of and contain genre conventions; ideally, it should play with these conventions or do something new with them.”

The self-referential nature of science fiction means that style and an extensive awareness of the genre’s history—rather than substance—separate a good sci-fi film from a merely average one. That’s not to say that sci-fi movies are devoid of meaning or broader concerns. The widely mocked B movies of the 1950s were certainly characterized by tackiness, but they also explored clever political allegory at the height of the Cold War.

In luring people back to Somerville year after year, the lineup of films is secondary to the festival’s unique communal experience, a theme that emerges repeatedly in conversations with both male and female audience members. (Contrary to any stereotypes of sci-fi as an exclusively male pursuit, the festival draws at least as many female attendees as male ones.) The festival is a richly interactive experience, with performers and audience competitions (years past have put on costume contests; this year featured an alien mating cry contest) interspersing the screenings.

Rick Juskiewicc has been making the trip north from Rochester, New York for 30 years, and now finds himself too old to sit through the entire show. Still, he is brought back every year by the camaraderie of “the Boston Marathon.” Malden resident Darryl Davis, a veteran of 20 festivals, feels similarly. “I come to meet the friends I’ve made that I see here every year,” he says. Even the movies themselves depend on the community atmosphere. “I wouldn’t watch the B movies at home by myself,” Simon says, “but here we all willingly suspend disbelief together.”

Given this community, it is no surprise that ties to the festival run deep, and the event is often a family affair. Miriam Dennis of Weymouth has come to every festival since 1986, and has brought her daughter Alex along for the last decade. And while the festival does attract the occasional lone eccentric—such as a self-proclaimed “magician” from Somerville who rotates the names Michael Fishman, Tuna Oddfellow and Fish the Magish—families are perhaps the most common type of attendee.

Not all fans are fortunate enough to have cooperative children, however. In his 30th year at the festival, Cambridge resident Barry Perlman lamented his inability to pass on the sci-fi tradition. “I try to bring my children, but they’re too square.”