Reformation Day, 31 October

Reformation Day falls on the thirty first of October, the date this rule records, commemorating the traditional account that Martin Luther posted his Ninety Five Theses on the door of the Castle Church in Wittenberg on this date in fifteen seventeen, an act of academic protest against the sale of indulgences that is conventionally treated as the opening event of the Protestant Reformation. Historians have questioned the literal detail of the door posting itself, since the earliest surviving accounts of that specific act postdate Luther's own lifetime, though there is no serious dispute that Luther circulated the Ninety Five Theses around this date in the autumn of fifteen seventeen, whatever the precise means of their first public posting.

Whatever its exact origin, the date's later significance is unambiguous, since the theses' circulation set off the sequence of events that produced the divided Western Christianity the calendar still reflects today. The observance itself is confined almost entirely to Lutheran and some other Protestant churches, marked with services recalling the Reformation's beginning and, in several countries with strong Lutheran heritage, treated as a civil holiday as well. Neither the Catholic Church nor the Orthodox churches observe the date at all, an absence that is itself the clearest marker of how the feast divides along exactly the confessional line its own history created, unlike nearly every other entry on this calendar shared across multiple Christian traditions.