Students in the Yale Symphony Orchestra began preparing for Halloween — albeit covertly — almost a year ago today.

Tonight, the YSO’s annual Halloween concert will begin at precisely 11:55 p.m. in Woolsey Hall, featuring, as it has for the past few years, a short pre-show followed by traditional sectional entrances, the showing of the silent movie and, of course, a smorgasbord of classical and popular tunes.

The concert, which has grown over the past two decades to become one of the most well-known Halloween events at Yale, is the orchestra’s most profitable performance, often filling the auditorium with more than 2,500 decked-out Yalies.

The details of the show’s specific plot are kept absolutely guarded. In fact, performers are under oath not to divulge it, Alejandro Bustillos ’11, who plays the main character in tonight’s show and serves as social chair of the orchestra, said. As part of the orchestra’s initiation, he said, all members swear four oaths, one of which promises to never speak of the contents of the Halloween show.“It’s never revealed before the actual performance,” he said.

Abiding by their oaths, few orchestra members told the News details about what to expect, but Caitlin McAuliffe ’09, the director of this year’s YSO show, hinted that it has unexpected elements.

“I can tell you that this year will be a different type of movie than in the past,” she said.

But the secrecy, the silent film and the costume-clad performers are all recent innovations to the show, said Alexander Kenigsburg ’09. The YSO Halloween shows began informally in the 1970s but started to include costumes and pranks in 1986. Somewhere along the way, the idea of a silent spoof movie was introduced, though the five members interviewed could not pinpoint the exact date.

And today, many traditions ­— including a usual cameo by Provost Peter Salovey, former dean of Yale College, and University President Richard Levin — are involved in the movie. This year, Marichal Gentry, dean of student affairs, has a prominent role but the YSO members interviewed would not discuss potential on-screen appearances by Mary Miller, the next dean of Yale College.

Every November, almost immediately after the Halloween performance, a committee convenes to discuss the possible topics for the following year’s movie. The movie is then written, perfected over the course of the year, and shot over the summer. They filmed this summer on location in Italy and here at Yale and around New Haven, according to Bustillos.

The show has grown in audience size and complexity since its inception. Each year, new multimedia effects are used, and the show has evolved into an interactive extravaganza. Kenigsburg said this year is no exception.

“I can’t tell you anything about the plot,” he said. “But I can tell you that it will feature a lot of new special effects.”

While for the audience, the Halloween show is one night of entertainment, for the orchestra, it is the culmination of months of practice and endless other responsibilities, said McAuliffe.

“Every since the beginning of the year, I’ve been at every single shoot,” she said. “I’ve never done anything like this in my life.”

Freshmen said they eagerly anticipate the show, ready to uncover the mystery.

“I’ve heard from upperclassmen that it’s a must,” said Kevin Adkisson ’12. “But I don’t know what to expect.”

Tickets will not be available at the door and must be purchased directly from YSO members or by phone.