One of the most fascinating aspects of Bach's Passions is the use of Lutheran chorales. They have a theological function-relating a song known by the entire congregation to an appropriate place in the Passion story. In the Matthew Passion, they also serve a structural role: Bach sets the same chorale, "O Sacred Head, Now Wounded" multiple times, with a different text and harmonization each time. For a long time, the St. Matthew Passion was thought to have been first performed on Good Friday Vespers, 1729, but it was probably first performed two years earlier, in 1727. This incorrect date was further solidified by a supposed centenary performance in 1829, under the baton of Felix Mendelssohn. A 102-year anniversary just doesn't have the same ring.