Day of the Week Calculator
Find the day of the week for any historical or future calendar date, plus whether its year is a leap year.
How It's Calculated
Formula
\text{dayOfWeek} = \text{weekday}(\text{UTC}(\text{year}, \text{month}, \text{day}))The date is first normalized to a UTC calendar-day value, then its weekday is read directly from that value. Because the normalization is done in UTC, which never observes daylight saving time, the result never depends on the local timezone of the machine running the calculation. Leap-year status uses the standard Gregorian rule: a year is a leap year if it is divisible by 4, except century years (divisible by 100), which must also be divisible by 400 to remain a leap year. This is why 1900 is not a leap year but 2000 is. An impossible date, such as Feb 30 or Apr 31, is rejected outright rather than silently rolled forward into a different, valid date.
Worked Examples
Known reference date: Jan 1, 1970
- Normalize Jan 1, 1970 to its UTC calendar-day value.
- Read the weekday from that value.
- 1970 is not divisible by 4, so it is not a leap year.
- Result: Thursday, not a leap year.
Century leap year: Feb 29, 2024
- 2024 is divisible by 4 and not a century year, so it is a leap year and Feb 29 is a valid date.
- Normalize Feb 29, 2024 to its UTC calendar-day value and read the weekday.
- Result: Thursday, a leap year.
Frequently Asked Questions
Why is 1900 not a leap year but 2000 is?
Century years (divisible by 100) are only leap years if they are also divisible by 400. 1900 divided by 400 is not a whole number, so it is not a leap year. 2000 divided by 400 is exactly 5, so it is a leap year.
What happens if I enter Feb 30 or another impossible date?
The calculator rejects it as invalid rather than rolling it forward into a different date (for example, treating Feb 30 as Mar 2).
Can I look up dates far in the past or future?
Yes. The weekday is computed with deterministic Gregorian-calendar arithmetic, so any valid calendar date from year 1 through year 9999 is supported.
Does this account for the historical switch from the Julian calendar?
No. This calculator applies the modern Gregorian calendar rules uniformly to every date, including dates before the Gregorian calendar's adoption, rather than modeling the historical Julian-to-Gregorian transition.