Day of the Week Calculator
Enter a day, month, and year to find the day of the week for that date. The calculator also checks whether the date is valid.
Check Day of the Week
Day of the Week Formula
Where:
- \(h\) — weekday number: 0 = Saturday, 1 = Sunday, through 6 = Friday
- \(q\) — day of the month
- \(m\) — adjusted month: March = 3, through December = 12; January = 13 and February = 14 of the previous year
- \(K\) — the last two digits of the adjusted year
- \(J\) — the century of the adjusted year, rounded down
This is Zeller’s congruence for the Gregorian calendar. It explains the weekday mathematically; the calculator itself uses JavaScript’s Date object to validate the date and obtain its weekday.
Day of the Week – Calculation Example
Example: find the weekday for April 10, 2025, the calculator’s default date. April is month 4, so \(q=10\), \(m=4\), \(K=25\), and \(J=20\).
\[h=\left(10+\left\lfloor\frac{13(4+1)}{5}\right\rfloor+25+\left\lfloor\frac{25}{4}\right\rfloor+\left\lfloor\frac{20}{4}\right\rfloor+5(20)\right)\bmod 7\]
\[h=(10+13+25+6+5+100)\bmod 7=159\bmod 7=5\]
Result: 5 corresponds to Thursday, so April 10, 2025, was a Thursday.
How to use the calculator
- Enter the calendar day in Day, from 1 to 31.
- Enter the month number in Month, from 1 for January to 12 for December.
- Enter the four-digit year in Year. For example, enter 2025 rather than 25.
- Read the weekday under Day of the Week. If the entered date does not exist, the result displays “Invalid Date.”
The starting values are Day 10, Month 4, and Year 2025. Leave them unchanged to check the example above, or replace all three values with your own date.
What the day of the week tells you
A date identifies a particular day in a calendar year; its weekday identifies its position in the repeating seven-day cycle. Knowing the weekday can help you check an appointment, look up the day on which an event occurred, or plan around workdays and weekends. The same day and month can fall on different weekdays in different years.
The weekday shifts because an ordinary year has 365 days, or 52 full weeks plus one day. A leap year has 366 days, or 52 weeks plus two days. For instance, a date after February may move two weekdays forward from one year to the next when February 29 falls between the dates.
The formula above is one way to calculate a Gregorian weekday by hand. January and February are treated as the final two months of the preceding year, which keeps the formula consistent around leap years. Its result is a number from 0 to 6, so the number must be converted to a weekday name. The calculator performs the date check and returns the name directly; it does not evaluate Zeller’s formula.
If you need to see a date in the context of its full month, use the Monthly Calendar Generator. To count the elapsed time between two dates rather than identify one weekday, use the Days Between Dates calculator.
Date validity and practical limitations
The calculator checks the complete date, not just whether each entry is within its field’s range. April has only 30 days, so April 31 is invalid even though 31 is allowed in the Day field. February 29 is valid only in a leap year. In the Gregorian calendar, a year divisible by 4 is usually a leap year, except century years must also be divisible by 400. Thus, February 29, 2000, is valid, but February 29, 1900, is not. For a quick check of a month’s length, see the Days in Month calculator.
The calculator relies on JavaScript’s Date object, which treats dates using the Gregorian calendar rather than switching to the Julian calendar for older historical dates. A date recorded under the Julian calendar may therefore require conversion before its historical weekday can be checked this way. Local time-zone rules can also affect unusual dates that were skipped when a region changed its civil time zone.
Use a full year rather than a one- or two-digit year. JavaScript interprets numeric years from 0 through 99 as 1900 through 1999 when creating a date; this calculator’s validity check will consequently reject those entries. The displayed weekday name follows your browser’s locale, so it may appear in a language other than English.
Frequently Asked Questions
How can I find a weekday by hand without using a formula?
Start with a date whose weekday you know, then count the days between the dates and divide by seven. The remainder tells you how many weekdays to move forward. For example, January 1, 2025, was a Wednesday. January 15 is 14 days later—exactly two weeks—so it was also a Wednesday. When counting across years, include February 29 if it occurred.
Will my birthday fall on the same weekday every year?
No. For a fixed date, the weekday usually moves forward by one from one year to the next, or by two if a February 29 falls between those dates. Your birthday will eventually return to the same weekday, but not on a fixed seven-year schedule. A 28-year pattern often works over stretches of the Gregorian calendar, but century leap-year exceptions can break it.
Why do different systems assign different numbers to the same weekday?
Weekday numbering depends on the convention. The ISO convention numbers Monday as 1 through Sunday as 7. JavaScript’s weekday number runs from Sunday = 0 through Saturday = 6, while Zeller’s congruence starts with Saturday = 0. The weekday name is the same; only its numeric label changes. Check the convention before using a weekday number in a spreadsheet or formula.
Can the same date have different weekdays in different time zones?
A calendar date has the same weekday wherever that date is in use, but one instant can have different local dates—and therefore different weekdays—in different time zones. For example, it may already be Monday in one location while it is still Sunday in another. If you are checking an event’s weekday, first convert the event’s time to the relevant local date.
Is a weekday the same thing as a business day?
No. A weekday is a position in the seven-day cycle; a business day depends on the place and purpose. Many businesses exclude Saturdays, Sundays, and public holidays, while others operate on weekends or follow different workweeks. This calculator identifies the weekday but does not determine whether banks, offices, or markets were open on that date.
Can I use a Gregorian weekday result for an old historical date?
Only if the date was recorded using the Gregorian calendar or has been converted to it. Countries adopted the Gregorian calendar at different times, and some dates were omitted during the changeover. A date written in a historical record may instead be Julian. Check which calendar applied in that location and period before comparing its recorded date with a Gregorian weekday result.
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