But wait, I thought it was Halloween. Well, there would have been no Halloween with ghosts and ghouls and candy galore if not for Reformation Day. There is some argument among scholars as to whether Halloween has solely pagan or Christian roots. It is indisputable, though, that without the November 1 holiday (holy day) of All Saints Day (instituted by Pope Gregory III in the eighth century to honor all saints and martyrs), there would have been no Halloween. The day before came to be known over time as All Hallows Eve, and eventually become Halloween.
So how does Reformation Day come into the picture? On this day in 1517 Martin Luther, dismayed by what he considered some corrupt practices in the Catholic Church, penned what has come to be called the 95 Thesis. He then nailed it to the door of the Wittenberg Castle church. What was a common means of entreating intellectual debate in the 16th Century, written in Latin and not the vernacular, ended up starting a Reformation that turned the Western world upside down.
A brief 5 Minutes in Church History by Stephen Nichols gives a concise explanation of why this day turned into Reformation Day.