Reformation Day is a Protestant Christian religious holiday celebrated on 31 October, alongside All Hallows’ Eve (Halloween) during the triduum of Allhallowtide, in remembrance of the onset of the Reformation.
According to Philip Melanchthon, 31 October 1517 was the day German Martin Luther nailed his Ninety-five Theses on the door of the All Saints’ Church in Wittenberg, Electorate of Saxony in the Holy Roman Empire. 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 that has never been proven. Available data suggest that 31 October was the day 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 (Melanchthon appears to be the only source for that) nailing of the Ninety-five Theses/grievances to All Saints’ Church’s door on the same date.
The holiday is a significant one for both Lutheran and Calvinist Churches, although other Protestant communities also tend to commemorate the day. The Roman Catholic Church recognized it only recently, and often sends its official representatives in ecumenical spirit to various commemoration events held by Protestants. It is lawfully and officially recognized in some states of Germany and sovereign countries of Slovenia and Chile. In addition, countries like Switzerland and Austria provide specifics in laws pertaining to Protestant churches, while not officially proclaiming it a nationwide holiday. (Source)