Why do we have leap years?
Earth's orbit takes about 365.24 days, not a whole number. An extra day roughly every four years keeps the calendar in step with the seasons.
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Science
Calendars
Where did the seven-day week come from?
The seven-day week has no single agreed origin. It likely draws on Mesopotamian sevens, the Jewish Sabbath cycle and the Roman planetary week, and Constantine made it official in AD 321.
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