5 We are not afraid to meet your scoff at the Sabbath, when you call it the fetters of Saturn. It is a silly and unmeaning expression, which occurred to you only because you are in the habit of worshipping the sun on what you call Sunday. What you call Sunday we call the Lord's day, and on it we do not worship the sun, but the Lord's resurrection. And in the same way, the fathers observed the rest of the Sabbath, not because they worshipped Saturn, but because it was incumbent at that time, for it was a shadow of things to come, as the apostle testifies. The Gentiles, of whom the apostle says that they "worshipped and served the creature rather than the Creator," gave the names of their gods to the days of the week.
And so far you do the same, except that you worship only the two brightest luminaries, and not the rest of the stars, as the Gentiles did. Besides, the Gentiles gave the names of their gods to the months. In honor of Romulus, whom they believed to be the son of Mars, they dedicated the first month to Mars, and called it March. The next month, April, is named not from any god, but from the word for opening, because the buds generally open in this month. The third month is called May, in honor of Maia the mother of Mercury.
The fourth is called June, from Juno. The rest to December used to be named according to their number. The fifth and sixth, however, got the names of July and August from men to whom divine honors were decreed; while the others, from September to December, continued to be named from their number. January, again, is named from Janus, and February from the rites of the Luperci called Februæ. Must we say that you worship the god Mars in the month of March? But that is the month in which you hold the feast you call Bema with great pomp.
But if you think it allowable to observe the month of March without thinking of Mars, why do you try to bring in the name of Saturn in connection with the rest of the seventh day enjoined in Scripture, merely because the Gentiles call the day Saturday? The Scripture name for the day is Sabbath, which means rest. Your scoff is as unreasonable as it is profane.
Source: Reply to Faustus the Manichaean (New Advent)