According to wikipedia:
On 31 October 1517, Martin Luther wrote to Albrecht, Archbishop of Mainz and Magdeburg, protesting the sale of indulgences. He enclosed in his letter a copy of his "Disputation of Martin Luther on the Power and Efficacy of Indulgences," which came to be known as The 95 Theses.
Almost 500 years ago according to His plan, the Lord allowed the seeds of what would change the course of the history of the Church and of the world to be planted. I do believe that for the entirety of the church age (even during the "Dark Ages") God has always had a remnant.
So whatever you are or are not doing or celebrating today:
One more thing :) James White's latest podcast was especially good in my opinion. He talks to turretinfan and then does some reformation history. If anyone has a spare hour, it is well worth the listen.
Happy Reformation Day!