People use a 7-day week because they have been born into a world where this is customary. In other words, the 7-day week has been received from earlier generations. It has a long history. When the Roman emperor Constantine made Christianity the state religion early in the 4th Century CE the 7-day week was officially associated with the Julian Calendar, and the association remained after the Julian Calendar was replaced by the Gregorian Calendar in the 16th Century CE.

The Christians received the 7-day week from the Jews. Their explanation for its use is that this was commanded by their god, named by them YHWH (using the Hebrew letters Yod-He-Vav-He). The Jewish Pentateuch (incorporated into the Old Testament of the Christian Bible) contains several injunctions attributed to YHWH which mention "a seventh day", upon which no "work" is to be done.

So clearly a 7-day week was in use at the time of Moses in the middle of the 2nd millennium BCE, but the 7-day week is much older than that, since it was also used by the Sumerians and Babylonians. Kerry Farmer remarks that "Some historians believe that around 2350 BC Sargon I, King of Akkad, having conquered Ur and the other cities of Sumeria, instituted a seven-day week, the first to be recorded."

In many European languages the names of the days of the week are derived from the names of planets/gods. According to Dr Kelley Ross the names for the planets/gods in Sumerian, Babylonian, Greek, Latin and English, with the English name of the corresponding day of the week in parentheses, are as follows:

Utu, Shamash, Helios, Sol, Sun (Sunday)
Nanna, Sin, Selene, Luna, Moon (Monday)
Gugalanna, Nergal, Ares, Mars, Mars (Tuesday)
Enki, Nabu, Hermes, Mercurius, Mercury (Wednesday)
Enlil, Marduk, Zeus, Iuppiter, Jupiter (Thursday)
Inanna, Ishtar, Aphrodite, Venus, Venus (Friday)
Ninurta, Ninurta, Kronos, Saturnus, Saturn (Saturday)

It is plausible to suppose that the association of planets and days of the week arose in prehistoric times as follows:

At some point in the evolution of humans, perhaps as far back as 100,000 years ago, they acquired sufficient intelligence to observe their environment and start to think about it. Obviously the night sky would have been of interest to early humans. The more intelligent among them would have observed that all of the luminous objects in the night sky maintained their positions relative to each other except for a few. Those that did not appeared to wander across the night sky (relative to the fixed stars), and thus eventually came to be called "wanderers". (The English word "planet" is derived from the Greek "planetes", which means exactly "wanderers".)

We may assume that tens of thousands of years ago humans did not think of the physical world as we do today, and in particular did not have an idea of the Earth as a large spherical object within a vast 3-dimensional space in which other large spherical objects moved. For them the nature of the luminous objects which they observed to wander along a band of the night sky, and the cause of their movement, was unknown. But since (by observation of the natural world) it was only living things which moved of themselves, it would be reasonable for early humans to assume that the wanderers, the planets, were living beings of some kind -- beings of a very unusual nature, what we might now call "gods".

So for early humans the planets were gods. And obviously the Sun and the Moon belonged to their company. So how many gods were there? As many as could be observed (perhaps more). In addition to the Sun and the Moon there were five others (what we now call Mercury, Venus, Mars, Jupiter and Saturn). If days somehow became associated with these gods then we have the basis for a period of seven days. Perhaps a particular god was venerated each successive day without a break, which would give rise to repeated periods of seven days.

It is plausible to suppose that the earliest calendars were simple tallies of days from one new moon to the next (where "new moon" means the reappearance of the moon after two or three days of invisibility). Bones with 29 and 30 scratches have been found which are at least 40,000 years old, suggesting (since a lunation is approximately 29.5 days) that the scratches were a record of days (or nights) in a lunation. This was probably the first attempt by humans to divide the sequence of days into periods. They would quickly have noted that four successive 7-day periods were almost, but not quite the number of days from one new moon to the next. This might have given rise to a calendar (such as is known to have been used by the Sumerians and Babylonians) in which the days of a lunation (a "month") were divided into four 7-day periods beginning with a new moon, followed by one or two days (not part of any 7-day period) until the next new moon.

The origin of the 7-day week is sometimes attributed to dividing the 29 or 30 days of a lunation by four, to get a number close to seven. But a concept of division, which we find easily understandable, is not a concept that we can attribute to the earliest thinking humans. Counting and addition may have been the most advanced mathematical concepts for many thousands of years before the idea of division (as a numerical operation) was discovered.

On the basis of this explanation of the development of the idea of the week it is obvious why there are seven days in a week: This is the number of visible planets plus the Sun and the Moon.

An immediate corollary is that there is nothing sacred (except in the minds of some people) about the fact that a week has seven days.

If, instead of an asteroid belt between Mars and Jupiter, there had been a planet, then there would have been six visible planets, not five, so the number of celestial entities would have been eight, not seven. In that case humans would have developed a week of eight days, not seven.

The Moon is thought by many astronomers to have been formed as a result of a collision of the Earth with a planet-sized object shortly after its formation over four billion years ago. If (assuming that is what happened) that collision had never occurred, and that no large body was subsequently captured by the Earth, then the Earth would have no moon, in which case the number of celestial entities would have been six, not seven. In that case humans would have invented a week of six days, not seven.

The planet Uranus was first observed by telescope in 1690 (by Flamsteed) but was recognized as a planet (by Herschel) only in 1781. Neptune was first observed in 1846. Had the solar system formed in such a way that these planets came close enough to Earth to be observable with the naked eye then the number of celestial entities would have been nine, and we would have a 9-day week. Actually the Maya had a 9-day week, with the days assigned to nine gods, called the Lords of the Night. One might speculate that the Maya knew (or were informed) that there were two more "gods" which were invisible (Pluto perhaps not being regarded as a fully accredited planet/god), though there is no other evidence supporting this idea.

The fact that humans have long used a week of seven days is thus the result of accident, namely, the fact that the solar system is the way it is, with five of the nine planets being sufficiently close to Earth to be visible with the naked eye.

The "sacredness" of the number seven is due to the association of the seven celestial beings (the visible planets plus the Sun and the Moon) with gods in the minds of early humans. This "sacredness" is thus illusory. And thus so too is the "sacredness" of the 7-day week. Accordingly there is no reason to preserve it, except from an exagerated respect for tradition. Those who adhere to some religion within which a 7-day week is given prominence will, of course, wish to retain a 7-day week in any new calendar. But for those whose minds are not constrained by religious (or astrological) tradition there is no reason to preserve a 7-day week. A week of 6 or 8 days may be considered on its merits, or even a week with a variable number of days. Such a week -- of 6, 7, 8 or 9 days, in accord with the variable length of quarter-lunations as they actually occur -- is part of a calendar invented in 2005 called the "Hermetic Lunar Week Calendar".