Chapter 23 of Leviticus states:

Seven weeks you shall count for yourself: from the time the sickle is put to the grain, you shall begin to count seven weeks. And you shall keep the Feast of Weeks for the Lord your God with a donation of your choosing, which you shall give the Lord your God.

We are now past the midpoint of the seven weeks between Passover and Shavuot—the name of the latter holiday being the plural of the Hebrew shavu’a, week. The word, coming from sheva, “seven,” also gives us shiv’ah, the seven days or mourning after a death.

The seven-day week is today universal, and there is a tendency to think of it as entirely natural phenomenon. After all, the division of the solar year into lunar months, each beginning with the appearance of the new moon, ending with the disappearance of the old one, and averaging 29.5 days, is natural. So is this cycle’s consisting visually of four parts: a first quarter in which the new moon is less than half the size of the full moon; a second quarter in which it goes on growing until the moon is full; and a third and fourth quarter in which it shrinks in reverse sequence until it vanishes. Since each of these quarters averages 7.375 days, what could be more obvious than dividing the month into four seven-day weeks with an extra day or two at the end of them?

Yet this was not obvious to the peoples of the past, though many of them did have lunar months, and some observed sub-monthly cycles as well. (The ancient Romans, for example, had an eight-day week called the nundinum that, based on a regular recurrence of market days, was unrelated to the waxing and waning of the moon.) As far as is known, it was only in the early-to-mid first millennium BCE that both the ancient Babylonians and the ancient Hebrews began dividing their lunar months into seven-day periods. Inasmuch as Babylonia was a Middle Eastern power that exerted great political and cultural influence at the time, and the Hebrews were a small people subject to this influence, these are unlikely to have been independent developments. In all probability, the seven-day week was a Babylonian invention that the Hebrews borrowed and developed within the framework of their own unique religious beliefs.

One of these beliefs, of course, was in a single God’s creation of the world in six days, after which He rested on a seventh called Shabbat. In this word, too, there appears to be a Babylonian influence, since the Babylonian calendar had a day called Shabbatum that fell regularly on the fourteenth or fifteenth day of every month—that is, on the day of the full moon that marked the end of the month’s second quarter. Called by an Akkadian cuneiform text ûm nûkh libbi (the Hebrew equivalent would be yom nu’aḥ ha-lev), “the day of rest of the heart,” Shabbatum did not have an importance anything like that which Shabbat came to occupy in Jewish life. Rather, it was a day, considered unlucky or ominous (in all likelihood because it marked the start of the moon’s waning), on which certain restrictions were imposed, some of which, such as eating food cooked over a fire or riding in a chariot, applied to the royal house alone.

Shabbatum occurred only once a month, but the seventh, twenty-first, and twenty-eighth days of the Babylonian month also had a special character that marked the turning points of the lunar cycle. Although no direct evidence has been found for this, it has been conjectured that each day of these seven-day periods was associated by the Babylonians, in whose thinking astrology played a major role, with the sun, moon, or one of the five visible planets—Mercury, Venus, Mars, Jupiter, and Saturn. At any rate, when the seven-day week was widely adopted in the Roman empire at the beginning of the Common Era, its days were assigned the Latin names of these same heavenly bodies.

Thus, the first day of the week was called in Latin Dies Solis, the Day of the Sun. The second was Dies Lunae, the Day of the Moon. The third was Dies Martis, the Day of Mars, followed by Dies Mercurii, Dies Iovis (the Day of Jove or Jupiter), Dies Veneris (the Day of Venus), and Dies Saturnis. The earliest attestation to these names is their presence in a graffito found at Pompei that cannot date later than the site’s destruction by a volcanic eruption in 79 CE, and they were ordered according to a system of reckoning that combined the 24-hour cycle of the day with each planet’s sidereal period—how long it took it to complete a circuit of the heavens and return to its original position vis-à-vis the fixed stars. Ancient Greek developed similar names for the seven days of the week at about the same time (heliou himera, “Sun’s Day,” selenis himera, “Moon’s Day,” etc.), and it may be that the Greek names preceded the Roman and served as their model.

Meanwhile, the Hebrews had their own seven-day week, which was already well-established in First Temple times, long centuries before it became so in the Roman empire. Its days did not, of course, have the names of pagan gods attached to them. Rather, they were numbered like the days in the account of Creation in the Bible—“First Day,” “Second Day,” “Third Day,” and so forth, up to the seventh day of Shabbat, which alone had a non-numerical name.

These two different ways of referring to the week’s days, one by name and one by number, have contended around the world practically up to this day. In many places, the clash between them belongs to the battle between Christianity’s pagan and Jewish heritages, and this will be the subject of our next, Shavuot column.