OTHER · DAY OF THE WEEK
Day of the Week Calculator
Find out what day of the week any date falls on — past, present, or future. Works correctly for leap years, century years, and dates as far back as the Gregorian calendar.
About This Calculator
Enter any date and instantly see which day of the week it falls on. Look up a historical event, plan a future date, or settle a debate about what day someone was born. Handles all edge cases — leap days, century years, and dates hundreds of years in the future or past.
How It Works
The calculator converts your date to a Julian Day Number (JDN), a continuous integer count of calendar days used by astronomers and standards bodies. The day of week follows directly from the JDN using modular arithmetic. This method avoids the DST ambiguities that affect simpler millisecond-based approaches and is accurate for any proleptic Gregorian date.
The Formula
Day of week = (JDN mod 7) + 1, where 1=Monday … 7=Sunday. The JDN is computed via the Richards algorithm from the input's UTC year, month, and day — the same algorithm used throughout this site for calendar arithmetic (Julian Day Number, Meeus 1998 §7).
Frequently Asked Questions
- Is 1900 a leap year?
- No. 1900 is divisible by 100 but not by 400, so it is not a leap year under the Gregorian calendar. February 28, 1900 was a Wednesday; there was no February 29, 1900.
- Is 2000 a leap year?
- Yes. 2000 is divisible by 400, making it a leap year (an exception to the century rule). February 29, 2000 was a Tuesday.
- How far back does this work?
- The calculator uses the proleptic Gregorian calendar — the modern calendar projected backwards before its 1582 adoption. Results are mathematically correct for any year, though dates before 1582 may differ from what local historical calendars recorded (Julian vs Gregorian discrepancy).
- What timezone does the calculator use?
- All dates are interpreted in UTC (midnight). For day-of-week purposes, the timezone only matters for dates at or very close to a timezone boundary — the vast majority of date lookups are unaffected.