In Europa Universalis II, the Reformation is a scripted event enabling the Protestant religion, which occurs sometime from 1515 through 1526. (Specifically, it occurs randomly on one of the 4000 days from Jan 1, 1515, to Feb 9, 1526.) In real history, the Reformation is generally thought to have started on 31st October 1517, when Martin Luther nailed his 95 theses to the church door in Wittenberg, Germany. See the wikipedia entry for the Protestant Reformation.