L'Harmonie des saisons, an early-music ensemble based in Granby, Québec, has sold out all seven of its Burlington concerts since its music director, Eric Milnes, first brought the lively and highly trained group to town two years ago. Now confident of the Queen City's taste for early music, particularly of the baroque era (roughly 1600 to 1750), Milnes has launched the first-ever Burlington Baroque Festival. Starting Thursday, September 26, it offers four days of music by the era's enduring stars: Claudio Monteverdi, George Frideric Handel, Antonio Vivaldi, Alessandro Marcello and Johann Sebastian Bach. A baroque music festival "has been part of my hopes and plans since I arrived" in Burlington in 2020, Milnes said. That year, he started directing the College Street Congregational Choir, and in 2022 he added director of the Vermont Choral Union to his job titles. Milnes assembled the festival's 14 vocal soloists, 27 musicians and 24-member chorus by combining L'Harmonie's professional members, who include invited early-music specialists from around the world, and 24 Vermont singers. It's a grand undertaking that wouldn't be out of place in a baroque-era church teeming with sculpted putti, gilded columns and lavishly painted surfaces. Alas, Burlington has no such treasure, so the festival will take place in the next-best venue: the clean-lined 1972 Cathedral Church of St. Paul. Baroque music is no rarity in the Green Mountains; Upper Valley Baroque of Hanover, N.H., often brings its concerts to Randolph, and various classical series occasionally feature works of that era. What distinguishes the festival is that its musicians will play period instruments and sing in historically appropriate vocal styles. Musicologists have painstakingly researched such performance techniques of the time over recent decades. Milnes, who earned his master's at the Juilliard School, has likewise spent decades implementing that research as director of numerous early-music ensembles in his native New York City, around the U.S. and abroad. Following historical practice, he directs from the harpsichord bench while playing. Two of L'Harmonie's albums have won JUNO Awards — the Canadian equivalent of the Grammys — for best classical album of the year. "I'm a hard-core early-music person, and Eric is as hard-core," said invited violinist Scott Metcalfe, who has performed with Milnes since 1986. Artistic director of the Boston early-music ensemble Blue Heron, Metcalfe will be the festival's concertmaster on Monteverdi's "Vespers of 1610"; Handel's two-act masque, or concert-style opera, Acis and Galatea;…