In addition to his responsibilities as music director of the St. Thomas and St. Nicholas churches in Leipzig, Johann Sebastian Bach was also responsible for performing in another role: as teacher. Bach served as the choir director and teacher of the students at the St. Thomas School, where he rehearsed the choir and also taught music theory twice a week. For the cantata that he was required to compose each week during the church calendar, he would form his choir by drawing together the forces of the soprano and alto choristers from the School as well as tenors and basses from the older students at the School and elsewhere in Leipzig. For major feast days, like Christmas, Bach would go all out with instrumentalists and vocalists by creating such works as the Passions and the Christmas Oratorio. The Christmas Oratorio isn't exactly your textbook definition of an "oratorio," though – it's actually a group of six cantatas for each of the six feast days of the Christmas season.