Eugene’s biggest annual musical happening, the Oregon Bach Festival, begins its 44th season, and founding artistic director Helmuth Rilling’s last, Friday night with one of the monuments of choral orchestral music — but one relatively rarely encountered in live performance. Though the composer himself wasn’t the only one who thought it among his finest works, Beethoven’s mighty Missa Solemnis is more admired than performed, in part because at nearly an hour and a half, it’s just too much — too long for most church services, even if you could fit the mammoth orchestra, colossal chorus and four vocal soloists in the sanctuary, or in the budget for a secular performance. Even in a form as traditional as a Christian religious service, Beethoven, ever the innovator, couldn’t help but push the boundaries, with extreme demands on the players and strange moments like martial drums and trumpets intruding on the last movement prayer for peace. All of which can make poor performances of the 1823 masterpiece (written at the same time as his Symphony No. 9 and no less ambitious) sound like a missa somnabulis and too tough for any but the best musicians to pull it off. Fortunately, that’s who’ll be performing it at the Hult Center’s Silva Concert Hall, including top soloists soprano Tamara Wilson, alto Roxana Constantinescu, tenor Nicholas Phan and bassist Ivan Garcia, guided by Rilling’s experienced hand.

Saturday’s afternoon free concert in the Hult lobby features the always exciting Oregon Percussion Ensemble, followed by a kid’s show by Czech multi-instrumentalist Radim Zenkl, who’ll also perform music of his homeland and surrounding areas that night in the Hult’s smaller Soreng Theater with keyboard-bass-percussion partner Leo Chern. Sunday’s chamber music concert at the UO’s Beall Hall features another visit from Los Angeles’ Bach’s Circle, featuring the sublime oboe-master Allan Vogel, who’s been gracing the festival with his creamy tone and incisive understanding of Bach’s music for more than three decades. They’ll play music for various combinations of violin, flute, harpsichord, bassoon and voice (soprano Elissa Johnston) of their namesake plus Johann Friedrich Fasch, Francois Couperin and Vivaldi. Anyone who thinks of Bach as a solemn old guy in a white wig really needs to hear this show’s Coffee Cantata about a young woman with a serious addiction.

Although Rilling will return to perform and teach at the festival from Germany for as long as he wants, there’s no telling how many more opportunities we’ll have to learn about some of music’s greatest masterpieces from the inside out, as taught by a musician who knows it as well as anyone alive. Rilling’s always illuminating Discovery Series of lecture demonstrations is a major part of his legacy, and this year’s exploration of Bach’s mighty St. John Passion begins on Monday afternoon at Beall Concert Hall and continues on Wednesday, July 3. In the same venue Monday night, one of the festival’s best concerts celebrates some of the 20th century’s finest and most listener-friendly classical composers with the emphasis happily on songs and chamber works from Francis Poulenc and Benjamin Britten. The starry lineup of singers and players includes tenor Nicholas Phan, pianist Robert Levin, soprano Tamara Wilson and more.

Wilson, alto Roxana Constantinescu and Britten’s great music return Tuesday night at the Hult in a program of opera arias and concert music that also features music by bicentennial opera deities Giuseppe Verdi and Richard Wagner. And Levin and his wife and fellow pianist Ya-Fei Chuang return Wednesday in a duo setting of more Wagner and tributes to him by Liszt, Faure and Chabrier.

In its first week, the festival includes music from the 18th, 19th and 20th centuries, but the festival’s Composers Symposium demonstrates that classical music is more than a museum — it’s a living tradition bursting with vital new sounds. On the afternoon of July 2 at Beall Hall, one of today’s leading choral conductors and choral music innovators, Craig Hella Johnson from Austin’s world-renowned Conspirare Chorus, will lead the chorus of the American Creators Ensemble in premieres of music for voices written by the emerging composers participating in this summer’s symposium.