The First Sunday Law 321 ADOn March 7, 321, Emperor Constantine ruled that Sunday, the Day of the Sun, was declared an official day of rest, on which markets were banned and public offices were closed.
The Decree of Constantine was immortalized in the Codex of Justinian:
“On the venerable day of the sun [Sunday] let the magistrates and people residing in cities rest, and let all workshops be closed.” (Codex Justinianus, lib.3, tit.12:3; translated by Phillip Schaff in History of the Christian Church, 1864, Vol.III, p.380).
The first day of the Roman Calendar was called dies solis, ‘Day of the Sun,’ because it was dedicated to Earth’s closet star. Our Monday is actually Moon Day or dies lunas, dedicated to Earth’s satellite. Each day had its honorary celestial deity. But Sunday was preeminent among the pagan culture and quickly adopted as a safe policy by Christians to avoid further persecution.
“Sunday, the first day of the week, was adopted by the early Christians as a day of worship...no regulations for its observance are laid down in the New Testament, nor indeed, is its observance even enjoined....” (The Schaff-Herzog Encyclopedia of Religious Knowledge, under the word “Sunday,” Vol.IV, pp.2259-60).
In 274 the Roman emperor Aurelian made Sol Invictus "the Unconquered Sun" an official Roman cult.
Notice the sun rays emerging from the heads of the god Sol Invictus on these coins from the time of Constantine
Lady Liberty wears the same solar crown as Sol Invictus and the chief sun god Helios
who rides his chariot daily across the sky drawn by four steeds.
“But it was during the rule of Constantine the Great that the cult of Deus Sol Invictus reached extraordinary heights, so that his reign was even spoken of as a Sun Emperorship. Constantine was the personification of Deus Sol Invictus on earth, and could consider the statue of the sun in the Forum bearing his name as a statue of himself” (Gaston H. Halsberghe, The Cult of Sol Invictus, 1972, p. 167)
While Constantine is best known for his Sunday Law, his motivation was not primarilly based on Christian theology.
“The devotion of Constantine was more peculiarly directed to the genius of the sun, the Apollo of Greek and Roman mythology; and he was pleased to be represented with the symbols of the god of light and poetry. The unerring shafts of that deity, the brightness of his eyes, his laurel wreath, immortal beauty, and elegant accomplishments, seem to point him out as the patron of a young hero. The altars of Apollo were crowned with the votive offerings of Constantine; and the credulous multitude were taught to believe that the emperor was permitted to behold with mortal eyes the visible majesty of their tutelar deity; and that, either waking or in a vision, he was blessed with the auspicious omens of a long and victorious reign. The sun was universally celebrated as the invincible guide and protector of Constantine.” (Edward Gibbon, Decline and Fall of the Roman Empire, Chapter 20, par. 3.)
“This legislation by Constantine probably bore no relation to Christianity; it appears, on the contrary, that the emperor, in his capacity as Pontifex Maximus, was only adding the day of the sun, the worship of which was then firmly established in the Roman Empire, to the other festival days of the sacred calendar.” (Hutton Webster, Ph.D., Rest Days: The Christian Sunday, The Jewish Sabbath, And Their Historical and Anthropological Prototypes, 1916, pp. 122, 123)
Closely linked to the weekly Sunday was the yearly Sun birth. Dies Natalis Invicti or the birthday of the Unconquered Sun (Dec. 25) was the principal day for this cult which had its roots laid in common with the Nimrod-Tammuz-Semirimis cults. It was on this day of the winter solstice that the sun began its ascent in the sky, the days began to get longer. This was interpreted as the resurrection of the solar deity. Constantine called this day the birthday of Christ.