Reformation Day is a religious holiday celebrated on October 31, alongside All Hallows' Eve (Halloween) during the triduum of Allhallowtide, in remembrance of the onset of the Reformation.
Traditionally, 31 October 1517 is widely held to be the day German monk Martin Luther nailed his Ninety-five Theses on the door of the All Saints' Church in Wittenberg, Electorate of Saxony within the Holy Roman Empire. Reliable evidence unambiguously confirming this event is not known of. Historians and other experts on the subject argue that Luther may have chosen All Hallows' Eve on purpose to get the attention of common people, although this has never been proven. Available data suggests that October 31 was when Luther sent his work to Albert of Brandenburg, the Archbishop of Mainz. This has been verified; nowadays, it is regarded as the start of the Reformation alongside the unconfirmed nailing of the Ninety-five Theses to All Saints' Church's door on the same date.
–Wikipedia