Day of the Week Calculator
Find the day of the week for any calendar date, past or future.
About the Day of the Week Calculator
"What day of the week was that?" comes up more than you'd think — checking whether your birthday fell on a weekend a particular year, confirming a historical date (was the moon landing really a Sunday?), or working out what day a future date lands on so you can tell if an event falls on a weekday. Mental calendar math gets unreliable more than a few months out in either direction, which is exactly when this is handiest.
Enter any calendar date and it returns the day of the week instantly, along with where that date falls in the year — its ISO day-of-week number and how far through the year it is. That second part is a quick way to see how close a date is to a quarter boundary or the year's midpoint without counting on your fingers.
How it’s calculated
The calculation uses the standard Gregorian calendar — the one in everyday use worldwide today — so results are exact for any real-world date. For dates before October 1582, when the Gregorian calendar was formally adopted (and adopted at different times in different countries after that), the result is a proleptic projection: it applies today's calendar rules backward, which won't match the Julian calendar dates actually in use at the time.
Frequently asked questions
What day of the week was I born?
Enter your birth date and the calculator returns the exact weekday it fell on, calculated using the standard Gregorian calendar.
Can this calculate the day of the week for a future date?
Yes — it works identically for any future or past date; the same calendar math applies regardless of which direction you're calculating.
Is this accurate for dates before 1582?
It's a mathematical projection rather than a historical record — the Gregorian calendar wasn't adopted until October 1582 (and at different times in different countries), so dates before that used the Julian calendar in real life, which drifts from the Gregorian by a growing number of days the further back you go.
How does this account for leap years?
Leap years are built directly into the Gregorian calendar rules the calculation follows, so they're automatically reflected in both the weekday result and the day-of-year figure — no separate adjustment is needed.
What's the difference between the day-of-week and day-of-year numbers shown?
The ISO day-of-week number ranks the weekday from 1 (Monday) to 7 (Sunday), while the day-of-year number counts how many days have elapsed since January 1st of that year — they answer different questions about the same date.
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