When did Christmas start being celebrated on 25 December?

Early Christians focused more on Easter than on the birth of Jesus, and the Gospels do not mention a date. A Roman calendar called the Chronograph of 354 records a celebration of the Nativity on 25 December in the year 336. One theory holds that the date was chosen to coincide with Roman winter festivals such as Saturnalia and the feast of Sol Invictus near the winter solstice. Another suggests early Christians calculated it by adding nine months to 25 March, the date they associated with the conception of Jesus. Many Orthodox churches still using the Julian calendar celebrate Christmas on 7 January.