Roaring ’round the globe for almost 20 years, Trans-Siberian Orchestra —TSO, for short — is the unlikeliest musical group to become not only one of the top concert acts ever, but a holiday tradition equal to Santa Claus himself.

TSO brings this symphonic-prog/classical-metal gift to the Chesapeake Energy Arena for two shows Saturday.

But, according to founder Paul O’Neill, it was the Christmas that almost wasn’t. Its first album was intended to be a rock opera about the Bolshevik Revolution that instead was reformatted for Broadway.

This led TSO to try something radically different, and the result was 1996’s Christmas Eve and Other Stories.

“It didn’t really sell,” O’Neill said. “But [our label] was like, ‘Paul, you’re on to something. Keep going,’ and when we finished Beethoven’s Last Night in ’99, we decided to tour and that’s when the band exploded.”

Five years later, TSO was among the world’s top 10 touring bands.

“It just seems to keep growing,” he said, “and our biggest fear now is, ‘Just don’t drop the ball.’” Dropping the ball seems to be the least of the act’s worries, as 2012 has been another “big year for TSO,” said O’Neill.

An industry veteran who has produced albums for Aerosmith, managed AC/DC and Ted Nugent, promoted concerts in Japan for Madonna and Sting, O’Neill takes changes in stride to help keep his shows fresh, and fans coming back.

This year’s concert is no different, switching focus to 2004’s The Lost Christmas Eve, because he thought its story of a Scrooge-like Wall Street banker “was more relevant about what’s going on in the world now.”

He also believes that The Lost Christmas Eve carries a deep, personal message, for himself and for the audience.

“I’ve always been fascinated by December 24th,” O’Neill said. “There’s just something about that day that allows you to undo mistakes you never thought you could undo. There’s something about Christmas Eve that’ll make you pick up that phone, call that person and say, ‘I can’t remember what we were fighting about,’ or ‘It was silly’ or ‘Let’s just hit reset and start over.’” Hope aside, TSO also promises to deliver entertainment.

“The Last Christmas Eve is a great story. The music is great. It rocks all the way through; it has a very poignant ending; and, like any of my stories, if it’s not happy, it’s not the ending,” O’Neill said. “If you want sad stories, just watch the news.”