What is Reformation Day?
On Oct 31, 1517, Martin Luther nailed a list of talking points against the sale of indulgences in the Catholic Church to the door of the Wittenberg Castle Church (the 16th century version of a bulletin board/blogspot). He wanted to debate whether or not it was proper for the church to offer ways for people to "buy their way" into God's good graces.
This discussion is viewed by many as the beginning of the Reformation, which reclaimed the biblical notion that salvation comes by grace through faith, not by works (Eph 2:8-10; Rom 1:16-17).
Reformation Day is a celebration of this time period when the gospel was rediscovered and cherished for the gift that it is.