For the first Christians of Jewish descent, the seventh day of the week and the day of rest was the Sabbath. Like all days of the Jewish calendar, it begins in the evening and lasts until sunrise. According to our calendar, the Sabbath begins on Friday night. The first Christians of non-Jewish origin switched the day of rest and prayer to a Roman day called dies solis (day of the Sun) because according to the Gospels Jesus was resurrected on the third day after the Sabbath. Today’s Sunday corresponds to that, and they began to call that day the day of the Lord. Therefore, in many Romance languages, the name for Sunday is associated with the word Lord: French dimanche, Italian domenica Spanish domingo.
In 321, Constantine I declared dies solis an obligatory holiday for both Christians and Mithraists: “All judges and inhabitants of cities and all artists should rest on the honorable day of the Sun.” Farmers were exempted from compulsory rest in case of urgent agricultural work. Constantine thus satisfied the worshipers of the two religions, and still portrayed himself on coins as the deity Sol invictus (Invincible Sun).