Reformation Sunday
Presbyterians celebrate the tradition that grounds their faith on Reformation Sunday. It is always the last Sunday in October, marking the occasion in 1517 when Martin Luther posted his 95 theses on the church door in Wittenberg, Germany.
As Christians, our faith began with the followers of Jesus. But the beginning of our Christian faith as Presbyterian is in the Reformation. The Reformation marks something significant for us in this part of the universal church: the coalescence of our focus on the Holy Spirit’s ongoing reform of us as a church and as individuals. Martin Luther helps us to understand and acknowledge that the institution of the church is not equal to God. We are called to worship the triune God alone, to claim Jesus Christ as Lord of the church, and to seek the ongoing sanctification of the Holy Spirit.  

Door of the Schlosskirche (castle church) in Wittenberg to which Luther is said to have nailed his 95 Theses on the 31st of October.
Source:  Wikipedia
For historical information go to Reformation Day in Wikipedia.