If someone were to ask us to divide the number 365 into smaller units, our first reaction would most likely be to divide it by five, giving us 73 components of five each.Why, then, is the solar year of 365 days divided into units of seven days each, rather than of five? Would it not have been more logical and more convenient for ancient peoples to have based their calendars on a five-day week?

Arithmetically speaking, the answer is obviously yes. Even so, we do not find a five-day week anywhere on the globe which has lasted more than a few years. While attempts were occasionally made to alter the seven day week, none lasted for any significant length of time. Significantly, not one of the innovative systems spread to other localities, much less embracing the globe, as does the seven-day week. For thousands of years, the overwhelming bulk of humanity has based its calculations of time on a week of seven days, and so it continues today in the twenty-first century.Over the thousand of years of its exile, the People of Israel have reached nearly every corner of the globe. No matter where it set foot, almost without exception, the week consisted of seven days, no more and no less, making the observance of Shabbat that much easier.

Let us stop a moment to take a look at the way Man measures time. Our calendar is built on days, weeks, months, and years. Of these four, the day, month and year are determined by astronomical phenomena. The length of the day is set according to the time the earth takes to complete one rotation on its axis. The month represents the time the moon needs to complete one full cycle, from new moon to new moon. Similarly, the year is the period of time which elapses as our planet speeds along its orbit around the sun and returns to the same point in space.

How strange! Even ancient man was able to observe and measure the movements of the sun and moon which determined the length of the day, the month, and the year. Only the week is not defined by some physical phenomenon which man can observe. The length of the day, month, and year was determined by the Creator when He set the heavenly bodies in the cosmos and gave each on its path and cycle of movement. Man observed these cycles, recorded them, and thus learned the length of each one.

Not so, the seven-day week. No matter how long or how intensely ancient man regarded the heavens, by day, or by night, there was no factor which dictated that the week have seven days.Man’s only source for the seven-day week is G-d’s explicit commandment to the Jewish People, firsthand, with no reference to the sun, moon, or stars. “Six days shall you labor, and do all your creative work, and on the seventh day, it shall be a Sabbath unto the L-rd, your G-d”(Exodus).There is no other historical source for a week based on seven days. Everyone acknowledges the Hebrew Bible as the source of the seven day week. This fact has never been contested.

As mentioned above, the number seven is not convenient, as it does not lend itself to incorporation in the solar year of 365 days. From time to time in the history of man, reformers proposed a change that would presumably make life easier. In ancient Egypt, the month was divided into three periods of ten days each, or five periods of six days each. In ancient Scandinavia, a five-day week was current for some time, while Assyria used a six-day week. At one time, the Romans used the interval between market days as the basis for their nine-day “week.” Some African tribes also used market day to determine the length of their week, which was sometimes four days, and sometimes six. In Polynesia and Micronesia, there were primitive tribes who had no concept of a week at all. Their month was one extended period of time determined by the cycles of the moon, and not subdivided into smaller units other than days.

In more recent times, following the French Revolution, an attempt was made to correlate a new unit of time, the ten-day “week”, with the solar calendar. At the behest of the leaders of the revolution, an astronomer, G. Romme, set up a calendar in which the week had ten days, for a total of thirty-six weeks or three hundred sixty days. The tenth day of each week was declared a day of rest.This left five or six extra days at the end of each year. These were added at the end of the year as a separate, short “week” in order to bring the total up to 365 or 366, as needed.

This arrangement made life extremely difficult for the Jewish community, as Shabbat fell on a different day each “week”, usually right in the middle of the business week. Nonetheless, it did not even occur to the Jew to make “amendments” to the seven-day week he had received at Sinai. 

At the time of the French Revolution, the Jewish community was not strong enough to object to the new calendar. Heaven assisted them by stirring the Church into opposition. After only twelve years of the new system, the calendar was again changed, re-instituting the seven-day week as before.

More recently, following the communist takeover of Russia, another attempt was made to tamper with the traditional calendar. Unlike the French, the communists were motivated by their determination to eradicate all traces of religious belief. They knew that tampering with the seven-day week would make it extremely difficult to persist in observing any kind of Sabbath, whether on Friday, Saturday, or Sunday. In an effort to curtail all religious observances, they instituted a five-day weekin the year 1927. Each month was to have exactly six “weeks” of five days each. In 1930, the system was changed to a six-day week, with five “weeks” a month. By 1940, the innovations were abandoned, and the Russian calendar reverted to a seven-day week, similar to that of the vast majority of nations on the face of the earth.

Despites these attempts to tamper with the seven-day week, the Jewish People remained faithful to their Shabbat as G-d gave it to them over three thousand years ago, at Sinai: “Six days you shall labor and do all your creative labor, and on the seventh day, (it shall be) a Sabbath unto the L-rd your G-d.”

From the onset, when G-d transformed us into a unique, united nation, charged with keeping His Torah, the Jewish People have calculated the week as having seven days. No matter where the winds of their exile took them, they clung steadfastly to their Shabbat, preserving it against all odds. In return, the Shabbat preserved the Jews as a separate nation, proud of their heritage and the role assigned to them thousands of years previously, with the giving of the Torah.