On 31 October 1517, Luther nailed his 95 theses concerning indulgences and the atonement on the door of the Castle Church of Wittenberg. Or at least this is what was believed for a long time. However, in 1961, the Roman Catholic Erwin Iserloh asserted that this cannot have happened. Since, there has been a lot of discussion about this.
The two nearly contemporary assertions about the supposed nailing of the 95 theses are from Georg Rörer and Melanchthon: however, in October 1517 both were outside Wittenberg. The claim of Melanchthon was only made after Luther’s death. Luther himself twice mentions that he, on October 31st 1517, sent letters with the 95 theses to the bishops of Mainz and Brandenburg, and that he, only when there was no reaction to them, published his theses. For these and other reasons it seems not unlikely that Luther indeed did not nail the 95 theses on the Castle Church of Wittenberg on October 31st 1517.