from the Christian History Institute:
ON THIS DAY, 31 October 1517 in Wittenberg, Germany one of the most famous events in Christian history occurred. A priest named Martin Luther nailed a challenge to a debate on the church door. It consisted of ninety-five statements, or theses, against the practice of indulgences-- theses which he was willing to defend. Scholars commonly issued such challenges in those days.
The brief account can be read here.