When was Bach's "Ein' feste Burg ist unser Gott, BWV 80" A mighty fortress is our god, meant to be played?

I mean, was it originally written for Carnival, or Lent? On wikipedia is says "Most of the cantatas that Bach composed in Leipzig were intended for Sunday services, but Ein' feste Burg was his contribution to a festival celebrating the Reformation"....what is this festival called?