The date of Jesus's birth is unknown. A spurious correspondence between Cyril of Jerusalem and Pope Julius I (337–352), quoted by John of Nikiu in the 9th century, is sometimes given as a source for a claim that, in the fourth century AD, Pope Julius I decreed that the birth of Jesus be celebrated on 25 December. Some speculate that the date was chosen to create a Christian replacement or alternative to Saturnalia, along the popular birthday festival of Sol Invictus, the Invincible Sun God, held on 25 December.
Around AD 200, Tertullian had berated Christians for continuing to celebrate the pagan Saturnalia festival. The Church may have hoped to attract more converts to Christianity by allowing them to continue to celebrate on the same day.
Christians in western Europe have continued to celebrate many of the traditional Saturnalia customs in association with Christmas.