Rabanus Maurus (776-856), abbot [clergyman/priest] of Fulda and later archbishop of Mainz, Germany, was seen as one of the greatest theologians of his age and probably the most cultured man of his time, and exceptionally learned in patristics [writings and lives of the “church fathers”]. He was also a zealous defender of the papacy and its teachings. In one of his works, he says:

“Pope Sylvester instructed the clergy to keep the feriae.... [But] from an old custom he called the first day [of the week] the ‘Lord's [day],’ on which the light was made in the beginning and also the resurrection of Christ is celebrated.”[1]

Rabanus Maurus didn’t mean to say that Sylvester was the first man who referred to the days of the week as feriae or who first started the observance of Sunday among Christians. He means that, according to the testimony of Roman Catholic writers, Sylvester confirmed those practices and made them official insofar as his church was concerned. Hence Rabanus says elsewhere in his writings:

“Pope Sylvester first among the Romans ordered that the names of the days [of the week], which they previously called after the name of their gods, that is, [the day] of the Sun, [the day] of the Moon, [the day] of Mars, [the day] of Mercury, [the day] of Jupiter, [the day] of Venus, [the day] of Saturn, they should call feriae thereafter, that is the first feria, the second feria, the third feria, the fourth feria, the fifth feria, the sixth feria, because that in the beginning of Genesis it is written that God said concerning each day: on the first, ‘Let there be light:’; on the second, ‘Let there be a firmament’; on the third, ‘Let the earth bring forth verdure’; etc. But he [Sylvester] ordered [them] to call the Sabbath by the ancient term of the law, [to call] the first feria the ‘Lord's day,’ because on it the Lord rose [from the dead], Moreover, the same pope decreed that the rest of the Sabbath should be transferred rather to the Lord's day [Sunday], in order that on that day we should rest from worldly works for the praise of God.”[2]

Note particularly, he says that "the same pope [Sylvester I] decreed that the rest of the Sabbath should be transferred rather to the Lord's day [Sunday]."[3] According to this statement, he was the first bishop to introduce the idea that the divinely appointed rest of the Sabbath day should be transferred to the first day of the week. This was an ecclesiastical decree or law. This is significant, especially in view of the fact that it was during Sylvester's pontificate that the emperor of Rome [Constantine] issued the first civil laws compelling men to rest from secular labor on Sunday, and that Eusebius, bishop of Caesarea, was the first theologian on record to present arguments, allegedly from the Scriptures, that Christ did transfer the rest of the Sabbath day to Sunday.