Day-of-Week Calculator

Worked example

January 1, 2027 falls on a Friday. Calendar systems compute this by counting the total number of days since a known reference date and taking the remainder after dividing by 7, using the proleptic Gregorian calendar for any date, including ones before its historical adoption. December 25, 2026 falls on a Friday too, a reminder that the same weekday can recur within a single year for dates that are exactly 52 weeks apart, since 52 weeks is exactly 364 days, one short of a common year's 365.

Common mistakes

• Assuming a date's weekday repeats on the same date every year. A common year is 365 days, which is not a multiple of 7, so the weekday for any fixed date shifts by one each year, and by two across a leap year.
• Applying the modern Gregorian calendar to a date from a region and era that still used the Julian calendar. The proleptic Gregorian calendar used here extends today's rules backward, which will not match a historical record from before a region's actual adoption date.

Limitations

This uses the proleptic Gregorian calendar throughout. It does not reflect the historical calendar actually in use in a given region on a very old date, since most of the world adopted the Gregorian calendar at different times across several centuries.