The 3-year-old Tucson Chamber Artists will turn a major corner this weekend when the choir performs Mozart's C-minor Mass.

It will mark the professional group's biggest concert to date, performing one of Mozart's most ambitious pieces and one of the grandest works in all of the classical repertoire.

"It's a major undertaking. It's the combined forces of the choir and the orchestra," conductor Eric Holtan said. "You can't whip these things together. The TCA has been thinking about this for months."

The C-minor Mass is a monumental work, and it's not done often; the last time it was performed in Tucson was by the University of Arizona choir and orchestra in 2003.

The Mass requires a full orchestral complement and a large choir, and a conductor with enough chutzpah to pull it off and keep an audience riveted for nearly 60 uninterrupted minutes.

Frankly, Holtan is a bit nervous, he acknowledged with a chuckle.

"It's a new experience for me, and in fact I've never conducted anything of this magnitude before," he said last week. "I've spent hours and hours studying this, but it's like being back in school: The more you learn, the more you realize how little you know."

Holtan is bringing in six additional vocalists to join his 24-member choir, and he has assembled a 26-piece orchestra featuring Tucson Symphony Orchestra players, helmed by concertmaster Steven Moeckel.

"When you tackle a work of this size, you go deeper into the recesses of Mozart's brain and his gifts and his skills," Holtan said. "This is one of the finest pieces he's ever written in terms of its beauty and its virtuosity."

The C-minor Mass demonstrates how Mozart was influenced by Neapolitan opera, Bach, Handel and the goings-on in Salzburg and Vienna. But like his monumental Requiem, the C-minor Mass was unfinished. It is missing, by some accounts, the Agnus Dei (Lamb of God), most of the movements of the Creed and some of the orchestration for the "Credo" and "Sanctus."

"It's not finished in the sense that there's huge chunks of text not set to music in the cradle," Holtan chimed in on the argument.

Holtan believes that Mozart never truly finished the Mass because he was overcome with grief over the loss of a son — one of his five children to die in infancy.

"That might explain why he was unable to go back and pick up where he left off," Holtan explained, then quickly noted that "it's pure conjecture, just as the Requiem leaves room for conjecture."

Holtan said his nervousness at the prospect of conducting the monumental work is mixed with excitement.

"I'm climbing the walls — I'm so excited," he said. "It doesn't get much better as far as this music. And then putting a choir and orchestra together is the ultimate."

The Tucson Chamber Artists feature veteran singers who've clocked in time with some of the best ensembles in the world, including the Dale Warland Singers, the London Symphony Orchestra Chorus, St. Olaf Choir, Santa Fe Desert Chorale, Westminster Symphonic Choir and Phoenix Bach Choir, according to press materials.

● Tucson Chamber Artists in concert

Featuring: The TCA Orchestra and guest concertmaster Steven Moeckel.

When: 7:30 p.m. Saturday and 3 p.m. Sunday.

Where: St. Michael and All Angels Episcopal Church, 602 N. Wilmot Road, Saturday; St. Andrew's Presbyterian Church, 7650 N. Paseo del Norte, Sunday.

Tickets: $15, $8 for students in advance; $20, $10 at the door; 881-3544.

The program: Mozart's C-minor Mass.