It's the biggest classical musical event of the year, watched live by millions of people around the world. One of the soloists canceled his U.S. orchestral debut to be there. The music will range from traditional to new, with a world premiere. Thousands of people will download and stream it the next day. Yet music won't be the primary focus.

The event, of course, is the wedding of Meghan Markle and Prince Harry.

"It's a phenomenal chance for us to promote classical music," said Christopher ­Warren-Green, one of the event's conductors.

Classical music comes to the fore at ceremonial occasions. For most people, only classical music will do, even if they don't otherwise listen to it. Brides turn to websites and magazines for advice on what they ought to play.

How much more fraught, then, is the selection of music for the royal wedding, which — as in the case of Prince William's wedding to Kate Middleton in 2011 — can be heard by an estimated 2 billion people. Charles, the Prince of Wales, is a music lover and played a major role in selecting music for the weddings of his sons, drawing on British ceremonial music going back centuries.

For Meghan and Harry's wedding, however, "there most definitely will be an American slant," Warren-Green said. He will be conducting an orchestra made up of members of various leading British ensembles. Meghan and Harry's will be his third royal wedding, and one of a long string of appearances for and with the royals he's made since 1981, when Prince Charles invited him to give a concert in Buckingham Palace.

He conducted at the wedding of Prince Charles and Camilla Parker-Bowles, and at Kate and William's wedding. He once taught Prince Charles to conduct Wagner's "Siegfried Idyll" as a present for his wife. He also recalls a private concert for the royal family on the queen's 80th birthday, leading a chamber orchestra in Handel's music, with a harpsichord dating from Handel's time, in a room in Kew Gardens in which Handel is likely to have performed himself.

Conducting a royal wedding is different from preparing a concert for one of his orchestras — the London Chamber Orchestra or the Charlotte (N.C.) Symphony Orchestra, where he has been music director since 2009. For one thing, he doesn't have much of a say in the repertoire.

Even the brides have had limited choice, although Middleton had a few picks for her 2011 wedding. One piece she wanted, as a recessional, was William Walton's "Crown Imperial," but the work was written as a coronation march, and the bridal pair were concerned this might violate protocol. They consulted Prince Charles, who told them not to worry. Still unsure, they asked the queen. She, too, said it didn't matter, and the music was part of the ceremony — playing as the bride and groom stopped to bow and curtsy to the queen on their way out of the church.

The repertoire for an English event is also markedly different from an American one. English ceremonial tradition is steeped in music, going back to Handel's day. (Who can forget soprano Kiri Te Kanawa at Charles and Diana's 1981 wedding singing "Let the Bright Seraphim" from Handel's oratorio "Samson" — a moment that sealed her stardom?)

Kate and William's wedding was rife with English compositions, from Hubert Parry to Walton to Peter Maxwell Davies to John Rutter, whom Westminster Abbey commissioned to write an anthem for the ceremony as a wedding present to the bride and groom.

Ceremonial music in the U.S. is more secular, and of more recent vintage. " 'America the Beautiful,' Sousa marches, and anything written by Gershwin, Bernstein or Copland," Warren-Green said. Our ceremonies are different, too: State offerings of classical music tend to be at Memorial Day or July 4th concerts, or funerals and memorials.

The repertoire for Meghan and Harry's wedding is still under wraps, although the performers have been announced. Warren-Green is one of a host of musicians under the overall leadership of Sir James Vivian, the director of music at St. George's Chapel, where the wedding will take place. The Kingdom Choir, a British gospel group, will perform, as will the Baroque trumpeter David Blackadder.

Much focus will be on 19-year-old cellist Sheku Kanneh-Mason, who was to have made his U.S. debut with the Los Angeles Chamber Orchestra on May 20. But Markle called him personally to invite him to play. An audience of 2 billion can't be beat.