While most people know today as Halloween, for millions of Christians October 31, 2018 is also the 501st anniversary of Reformation Day. Here are five facts about the Protestant holiday:

1. Reformation Day celebrates Martin Luther’s nailing his ninety-five theses to the church door Wittenberg, Germany on October 31, 1517. (Some scholars debate whether he posted them to the door then, later in November, or whether he even posted them at all.) By posting them to the church door—which was similar to a public bulletin board—Luther intended to start a debate among his fellow academics.

2. While the historical date for the observance of Reformation is October 31st, most churches celebrate it on the last Sunday in October. The only countries in which Reformation Day is a national holiday areChile and Slovenia. (Though in Chile it’s calledDía Nacional de las Iglesias Evangélicas y Protestantes—National Day of the Evangelical and Protestant Churches.)

3. On the first “Reformation Day”, when heposted the ninety-five theses, Luther was not yet a “Protestant.” (He would later say that at the time he was mitted Catholic whowould have murdered—or at least been willing to see mitted—in the name of the Pope.)

4. The theses that sparked the Reformation were not particularly radical, and key Lutheran doctrines, such as justification by grace through faith alone, were not included. The theses were mainly about the selling of indulgences, which theCatholic Church defines as“a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints.” At the time, believers could pay for an indulgence to reduce the punishment of someone believed to be in Purgatory.

5. On this same day, Luther sent aletter to the Archbishop of Mainz, under whom, as it says in the letter, “Papal indulgences for the building of St. Peter’s are circulating under your most distinguished name.” Soon after this letter was sent, the theses were circulating among the intellectual elite. Although the rapid spread of the ideas that launched the Protestant Reformation are often attributed to the printing press (which had been around for more than 60 years), somescholars now saythe document went “viral” because of the “wider system of media sharing along social networks—what is called ‘social media’ today.”