Weekdays Between Two Dates

The seven-day week is a Babylonian inheritance, refined through the Hebrew, Greek, and Roman calendars and fixed in Christian-era Europe by Emperor Constantine's edict of 7 March 321, which made Sunday (dies Solis) the legal day of rest. The seven-day cycle was independently developed in ancient Chinese and Indian calendars, suggesting a common astronomical basis - the seven classical planets (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn) visible to the naked eye.

The English names of the weekdays preserve the Norse and Germanic gods (Tiw = Tuesday, Woden = Wednesday, Thor = Thursday, Frigg = Friday) substituted for the Roman planet-gods during the Anglo-Saxon adoption of the seven-day Roman planetary week around 300 CE. Latin Mars became English Tiw and so on. Monday and Saturday retain the Moon and Saturn directly; Sunday retains the Sun.

The 5-day Mon-Fri workweek was popularised by Henry Ford in 1926 when Ford Motor Company became one of the first major US employers to give workers a two-day weekend. The Fair Labor Standards Act of 1938 then codified the 40-hour workweek (8 hours x 5 weekdays) as the federal standard above which overtime pay was required. The two-day weekend (Saturday + Sunday) was the natural consequence.

Saturday joined Sunday as a day off largely for religious reasons - Jewish workers observed Sabbath on Saturday, and US labour leaders successfully campaigned for accommodation. Today Sunday remains the dominant religious day of rest in Christian-majority countries, Friday in Muslim-majority countries (which often have Fri-Sat weekends, e.g., UAE until 2022, Iran, Saudi Arabia), and Saturday in Jewish-majority Israel (Israeli weekend is Fri-Sat).

ISO 8601 standardised the week-numbering and weekday-numbering conventions in 1988. ISO 8601 defines Monday as day 1 of the week (1=Monday through 7=Sunday) - opposite to the US convention where Sunday = day 1 of the week. The week-numbering scheme (week 1 is the week containing the first Thursday of the year) is used in European calendars, ERP systems, and most scheduling software.

Counting Mon-Fri weekdays between two dates is the foundational primitive for SLA contracts, court deadlines, school days, payroll periods, and project planning. The US federal court system counts business days excluding weekends and 11 federal holidays. Many SaaS providers (Salesforce, Asana, Linear) use weekday counts for project Gantt charts so a 10-day task spans exactly two work weeks.

Across the world, the workweek varies. UAE moved to Mon-Fri in 2022 to align with global markets, abandoning the historical Sun-Thu workweek. Israel runs Sun-Thu. Brunei runs Mon-Thu and Sat. Most of Africa, Asia, Australasia, Europe, and the Americas use Mon-Fri. This calculator's default is Mon-Fri but the breakdown shows how many of each weekday name fell in the range so non-standard weekend rotations can be derived.