Why do Lutheran churches observe Reformation Day on October 31?

Lutheran churches observe Reformation Day on October 31 to commemorate the anniversary of the day in 1517 when Martin Luther is traditionally said to have posted his Ninety-five Theses on the door of the Castle Church in Wittenberg, Germany. This event is identified by Protestants as the beginning of the Reformation, a movement that permanently reshaped Christianity.12

While early Lutheran churches commemorated the Reformation on various dates, the centennial celebrations of 1617 focused attention on October 31. In 1667, Elector of Saxony John George II officially decreed this date as the annual Reformation Day in Saxony, a practice that was gradually adopted by other German Lutheran and Union territorial churches. Today, the day serves as a time for reflection on the Reformation and its theological impact, and in some regions, it is also observed as a religious alternative to Halloween.1