Week
Non-SI calendar unit · quantity: time
A day is the planet turning. A month is the Moon going round. A year is the Earth going round. The week answers to no motion in the sky: seven days divide neither the month nor the year without a remainder, and no luminary returns to its starting point after seven days. It is the one calendar cycle held up by agreement alone — which is exactly why it was twice declared arbitrary and abolished: in France in 1793 and in the USSR in 1929. Both times it came back.
01 · Definition
A week is a period of seven days, that is 604 800 seconds exactly. The unit is not part of the SI, and it is not even on the list of non-SI units accepted for use with the SI: the minute, the hour and the day are there, the week is not. It lives in the calendar, in labour law and in medicine, but not in metrology.
The only astronomical foothold is the phases of the Moon: 7.38 days pass from new moon to first quarter. But the lunar month is 29.53 days and does not divide into four whole weeks: a day and a half is left over. Any calendar that ties weeks to the Moon starts each month with a fresh week and a stub — which is exactly what was done in Mesopotamia, where the seven is thought to have come from: the seven moving lights of the sky known to the naked eye — the Sun, the Moon, Mars, Mercury, Jupiter, Venus, Saturn.
Hence the consequence that sets the week apart from every other unit of time: it synchronises with nothing. 365 does not divide by 7 — a year always shifts the weekday by one, a leap year by two. The month does not divide by 7 either, so the first of the month wanders across the whole week. The week simply walks its own pace through the calendar, never once interrupted in all of history — not even at the switch to the Gregorian calendar, when ten days were struck out of October 1582 and the order of weekdays was kept.
Since the basis is an agreement, one can agree otherwise. And so people did: eight days in Rome, ten in France, five in the USSR, four among the Igbo, five in Java. Below is a bench where you can rewrite that agreement and see what comes out.
Rewrite the agreement: how many days are in the week and how many groups the population is split into. One group means a day off shared by everyone. More than one and the days off slide, as in the Soviet continuous week of 1929. The grid below is a month of thirty days; the two rows under it are you and your spouse from another group.
This is the closest the week comes to astronomy — and it is not close enough. Four quarters make 29.53 days, four weeks make 28: a day and a half apart every month, eighteen days a year. A lunar week starts over each month with a stub at the end, so no continuous count can be built from it.
02 · Conversion
Enter a value — the sheet will convert it
The week takes no prefixes: “kiloweek” appears in no standard. It goes readily into a denominator, though — 40 hours a week, two doses a week, millimetres of rain per week. And where Russian has a single word for “a week and a half”, English has to spell it out — a small sign of how differently the two languages count weeks.
The third family in the table is about which day the week starts on. The answer depends on the country, and it is no trifle: the same date falls into different weeks, and the week number in a report differs by one.
In Portuguese and Greek the days of the week are simply numbered, but the count starts from Sunday: segunda-feira — “the second day” — is Monday. So in a Portuguese calendar Monday cannot be first: it is second by name. Russian day names follow the same logic but count from Monday: Tuesday is the second, Thursday the fourth, Friday the fifth, and Wednesday is “the middle” — the middle of a seven-day week that begins on Monday.
| Note | |||
|---|---|---|---|
| {u} | {name} | {value} | {note} |
03 · Orders of magnitude
a life measured in something other than years04 · Measuring instruments
What keeps count of weeks
Seven columns for a whole year — the most widespread measuring instrument in human history. It measures nothing natural: it shows where the agreement currently stands. The red column on the right is an agreement too, and a young one: a universal Sunday off was written into Russian law only in 1897.
A disc with windows that gives the weekday for any date. It works because the Gregorian calendar closes on itself: 400 years are 146 097 days, and that divides by seven exactly. After 400 years the calendar repeats in full, weekdays included.
To make a clock show the weekday, watchmakers fit a wheel with a tooth count divisible by seven and a lever that pushes it once a day. No other calendar function needs a seven: months have 31 and 30, the year has 12 and 4. Seven is the one number in the movement that did not come from the sky but was put there by people.
In reporting the week became a proper numbered scale: 2026-W35-4. The ISO rule: the first week of the year is the one containing the first Thursday. In the American tradition it is the one containing 1 January. Hence the difference of one and the famous year-boundary bugs in reports.
05 · Writing rules
wk, wks and the date with a W
The week has no international SI symbol — technical texts write wk. The capital W is taken by the watt. In ISO 8601 calendar notation the week is marked by a capital W with a four-digit year: 2026-W35-4, where 4 is Thursday. Weeks are numbered from 01 to 52 or 53; there is no week zero.
A particular trap is the year in a week date. The week has a year of its own: 1 January 2027 falls on a Friday, and in ISO notation that day is 2026-W53-5 — it belongs to the previous year. Systems that take the calendar year and the week number from different functions draw, at this boundary, a report for week 53 of a year that does not exist.
06 · Neighbouring units
The week sits between the day and the month, and both links are ragged: a month holds 4.35 weeks, a year 52 weeks and one day. The week fits exactly only into the 400-year Gregorian cycle — and that is the single exact ratio it has.
1/7 of a week
604 800 s
4.35 weeks
That closure has a side conclusion, easy to check by counting: over 400 years the thirteenth falls on a Friday 688 times — more often than on any other weekday. No mystery about it: months of unequal length simply spread unevenly over the seven, and the cycle is finite.
07 · Historical section
Two attempts to abolish the seven
Rome lived by an eight-day market cycle: letters A to H in the calendar, and every ninth day by Roman reckoning was market day, when farmers came into town. The seven-day planetary week arrived later and shared the calendar with the nundinae for a couple of centuries before pushing them out.
The strange order of the days — Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn — matches neither brightness nor distance. It comes out if you hand the 24 hours of the day to the seven luminaries in a circle: the next day begins with the planet three places on. Hence Saturn’s Saturday, and Sunday, and Lundi.
The Republican calendar replaced the week with the decade: three decades to a month, thirty days exactly, rest on the décadi — one day in ten instead of one in seven. Peasants and workers did that arithmetic quickly. Napoleon brought the seven-day week back on 1 January 1806; the calendar did not live to be thirteen.
A five-day week with a sliding day off: workers were split into five groups, each with its own rest day, and the factories never stopped. Two years later came the six-day week with a shared day off tied to dates, and on 26 June 1940 the seven-day week returned, Sunday included.
A unit with no standard and no phenomenon
The metre had a meridian, the second had the day, the kilogram had a weight. The week has nothing: no natural process it measures and no artefact to check it against. There is only one way to verify a week — ask your neighbour what day it is. That is precisely why it cannot be lost and cannot be spoiled: the count of weeks was broken neither by the Gregorian reform of 1582, when ten days were cut out of the calendar, nor by Russia’s switch to the new style in 1918. The date moved; the weekdays did not.
Both attempts to abolish the seven failed for the same reason: the week is not a physical quantity but a social synchroniser. Its whole value lies in everyone counting alike. The continuous week broke exactly that: the factory ran without stopping, but a husband in the first group and a wife in the third had not one free day in common, and “the weekend” stopped being a word you could say to everybody at once. A unit with no standard in nature rests on consent — and collapses the moment consent is repealed by decree.
Catalogue · units of measurement
A passport for every quantity
Seven base SI units, twenty-two derived ones with their own names, and units outside the system without which neither engineering nor the calendar would work.