On October 31, 1517, Martin Luther posted his 95 theses on the church door paving the way for the Protestant Reformation. Apparently some churches go as far as to celebrate Reformation Day on the Sunday before to Halloween (not Christian Science churches of course).
Later Bach composed some special music for the occasion.
What is scarier than a ghost or goblin asking for candy? A German Monk demanding religious reform in Latin!
Update: I made this mistake of mentioning this factoid to my husband and promptly got a lecture about the gregorian calendar.