How Is the Day of the Week Calculated?

The seven-day week has run continuously and without interruption for thousands of years, which means the weekday for any calendar date — past or future — can be calculated exactly rather than estimated.

The basic idea

Every date can be converted into a day count from a fixed reference point. Taking the remainder of that count divided by seven gives the weekday, because the pattern of weekdays repeats every seven days. Calendar software (including this site) typically does this by converting a date into a single serial number and using modular arithmetic, rather than manually counting.

Try it

Use the Day of the Week tool to check any date — it works identically for a date from decades ago or decades in the future.