On April 5 of this year, Trans-Siberian Orchestra changed forever.

That was the day Paul O’Neill, the founder of the combination progressive rock band and orchestra, passed away from an accidental reaction to prescribed medications he’d been taking. As the main songwriter and the man who developed the stories and concepts behind each album from TSO, the group’s future was altered with O’Neill’s passing.

But Al Pitrelli, who serves as musical director for one of the two TSO units that bring the band’s Christmas tours to arenas nationwide, said fans won’t see TSO fade away any time soon. You can catch them next Wednesday, Dec. 6 for an afternoon and evening show.

Pitrelli has been involved since the early days of TSO. The group was designed to carry on without O’Neill or any of its main music contributors. The group was not based around a frontman or primary instrumentalist. Instead, O’Neill cast musicians, and especially singers, to fit whatever song he was writing.

What isn’t so certain is the future of several album projects that were in progress when O’Neill passed away. But Pitrelli expects that the projects will come to fruition with the help of O’Neill’s writing partner Jon Oliva and O’Neill’s wife, Desiree, who are still involved.

“Paul and his writing partner, Jon Oliva, have quite a few projects in the works. There are a few records that we had been recording,” Pitrelli said, mentioning “Romanoff: When Kings Must Whisper,” “Streets: A Rock Opera” and “Gutter Ballet,” as three rock operas that are in progress. “(TSO) was their child that they gave birth to years and years and years ago. It’s so nice to know that the family is going to carry on the family’s legacy.”

What TSO accomplished during O’Neill’s life was already impressive and quite unique within the music industry.

It started when O’Neill first began producing for the rock band Savatage, which at the time included Oliva, Pitrelli and Robert Kinkel. O’Neill already had the idea of combining a rock band and orchestra to make rock operas, drawing on stories from historical figures and events. Atlantic Records bought into O’Neill’s vision and provided him the finances and creative freedom to make it happen. And O’Neill brought along Oliva, Pitrelli and Kinkel to form his creative team.

O’Neill first hit paydirt by breaking into the Christmas market with the elaborate albums “Christmas Eve and Other Stories” in 1996, followed by “The Christmas Attic” in 1998. The first album eventually became a triple-platinum hit. The track “Christmas Eve Sarajevo,” hit the top 50 on “Billboard” magazine’s Hot 100 singles chart three straight holidays as it accumulated 1.3 million downloads. “The Christmas Attic” has gone double platinum and the trilogy was completed with the double-platinum “The Ghosts of Christmas Eve” in 2004.

In 1999, TSO went on its first holiday tour playing arenas. The concert included an innovative, eye-popping show filled with pyrotechnics, lighting effects and other visual bells and whistles that almost made a Kiss concert seem stark by comparison.

This year’s concert will be anchored by a performance of the compilation album “The Ghosts of Christmas Eve: The Best of TSO and More.”

“We’ve become such a tradition. We’ve become to people what ‘It’s a Wonderful Life’ or ‘Miracle on 34th Street’ was to me when I was kid,” Pitrelli said. “This is something that people have latched onto and made part of their holidays. It’s a lot of fun.”