Luther “wrote [the 95] theses on indulgences and posted them on the church of All Saints on 31 October 1517,” wrote Phillip Melanchthon. Protestants have celebrated this event since the late 16th century, and October 31th became Reformation Day in the Protestant areas of Germany in the early 18th century.
The famous composer J. S. Bach wrote cantatas for Reformation Day. For the one in 1727, he wrote the following cantata, based on Luther’s A Mighty Fortress is our God (“Ein feste Burg ist unser Gott”).
And for the Reformation Day of 1725, he wrote this one.
Let us, with Bach, rejoice and be glad.
Post Tenebras Lux