Week Calculator

Converting between days and weeks is integer division by seven, but the interesting behaviour lies in what does not divide evenly. A common year of 365 days is 52 weeks plus one day, and a leap year is 52 weeks plus two, which is why a given date advances one weekday each year and two across a leap. Over the full Gregorian cycle of 400 years there are 146,097 days, and because that figure is exactly divisible by seven it equals 20,871 whole weeks, so the pattern of weekdays repeats every four centuries. The average year contains 52.1775 weeks. Business weeks are a different unit, derived here by counting only Monday through Friday and dividing by five. Two limitations follow. The business-day count ignores public holidays entirely, so a fourteen-day span containing Christmas will be overstated by one or more working days, and it assumes a Monday-to-Friday working week, which does not hold where Friday or Sunday is a rest day. Separately, week-based intervals are not interchangeable with month-based ones: four weeks is 28 days, whereas a month averages 30.44 days, which is why fortnightly and monthly payrolls produce annual period counts of 26 and 12. Under ISO 8601 weeks begin on Monday and week 1 is the week containing the first Thursday of January, so a year has 52 or 53 numbered weeks, with long years occurring 71 times in each 400-year cycle. The Fair Labor Standards Act defines a workweek as a fixed, regularly recurring period of 168 hours that may begin on any day and hour.

The seven-day week has no astronomical basis, unlike the day, month and year, and its persistence is a matter of transmitted custom. Two streams converged: a Babylonian tradition associating days with the seven classical planets, and the Jewish Sabbath cycle, which fixed a recurring seventh day of rest. The planetary week spread through the Hellenistic world and the Roman Empire, and Constantine gave it official standing on 7 March 321 CE by making dies Solis a day of rest throughout the empire. Attempts to replace it have consistently failed. The French Republican calendar in force from 1793 to 1805 divided months into three decades of ten days, and the Soviet Union introduced a continuous five-day week in 1929 and a six-day week in 1931 to keep factories running without a common rest day, abandoning both and restoring the seven-day week in 1940. Numbering weeks rather than merely naming days is a much later administrative development driven by industry and logistics; the international standard was issued as ISO 2015 in 1976 and later absorbed into ISO 8601, first published in 1988 and revised in 2000, 2004 and again as ISO 8601-1 in 2019. On the employment side, the statutory American definition of a workweek and the forty-hour overtime threshold both come from the Fair Labor Standards Act of 1938, while the European Union's Working Time Directive of 1993 set an average forty-eight hour ceiling including overtime.