The thirty-first day of October.
- October 31 is Halloween. Although the name comes from the Christian holiday of All Hallow's Eve (the night before All Hallow's Day or All Saints' Day), many Christians see a pagan element in popular celebrations of Halloween.
- Reformation Day
- 1517: Martin Luther posts the 95 Theses on the door of the Castle Church in Wittenberg, Germany.
- 1731: Archbishop Leopold von Firmianhe, Catholic ruler of Salzburg, Austria, issued an edict expelling as many as 20,000 Lutherans from his principality.
- 1992: Pope John Paul II apologizes for the 1633 heresy trial against Galileo Galilei.
- 1999: The Lutheran World Federation and the Roman Catholic Church sign the Joint Declaration on the Doctrine of Justification in Augsburg, Germany.
- Apostle Narcissus (Eastern Christianity).
- Apostle Stachys (Eastern Christianity).
- All Hallow's Eve, also known as All Saint's Eve.
- Apostle Aristobulus