Seven-day week

The seven-day week is a repeating cycle of seven days used to organize time independently of months and years. Its modern form is widely used internationally and is not synchronized with the astronomical year.

Babylonian calendar: A lunisolar calendar used in ancient Babylonia, with months beginning near the new moon. Babylonian astronomical and calendrical practice helped establish seven-day cycles in the ancient Near East.
Weekday: One of the seven named days in a recurring weekly cycle. Weekdays are the cycle's individual positions, repeated in the same order.
Constantine the Great: Roman emperor from 306 to 337 CE who made Christianity favored and issued major imperial reforms. His 321 CE Sunday rest law gave the weekly cycle a place in Roman civil life.
French Republican calendar: A calendar used in France from 1793 to 1805, dividing each month into three ten-day periods. Its décades replaced the familiar seven-day rhythm with a ten-day cycle.
Shabbat: The Jewish day of rest, observed from Friday sunset to Saturday nightfall. Jewish weekly observance preserved a continuous seven-day cycle across generations.
Sunday: The day commonly treated as the first or last day of the week, depending on convention. Its placement illustrates that the cycle's starting point is a social convention.
Moses: A central figure in Jewish tradition, associated with the exodus and the receiving of the Torah. The biblical creation narrative links the seventh day of the week with rest.
Decimal time: A timekeeping system that divides units into powers of ten rather than traditional groupings. Decimal calendar reforms challenged the inherited week as an arbitrary unit.
Roman calendar: The system of civil timekeeping used in ancient Rome, revised several times before the Julian calendar. Roman adoption helped carry the seven-day cycle across the empire.
ISO week date: A date system that numbers weeks and weekdays, with weeks beginning Monday and week one defined by Thursday. It provides a formal rule for assigning dates to continuous weekly cycles.

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