Day of the Week Calculator

Related Calculators

Calendar system: The date rules used to interpret your Date. This calculator supports Gregorian, Julian, and Auto.

Date: The year, month, and day you want to test.

Day of the week: The weekday name for the entered date, such as Monday or Thursday.

Day number in week: The weekday's position from 1 to 7 based on Week starts on.

Week starts on: The rule for which day counts as day 1 of the week. It changes the number, not the weekday name.

Leap year status: Whether that year has an extra day in February under the chosen calendar rules.

Day of year: How far the date is from January 1 within that year. January 1 is day 1.

Calendar note: A short explanation of which calendar rule was used and whether historical caution matters.

Mistake: Picking the wrong Calendar system for a historical date and then trusting the Day of the week answer without checking context.
Fix: For older records, compare Auto , Gregorian , and Julian , then read the Calendar note before using the result.

Mistake: Entering a date like 2025-02-29 or 2026-04-31 in Date .
Fix: Enter a real calendar date only. If February 29 is rejected, check the Leap year status for that year and calendar system.

Mistake: Expecting Week starts on to change the weekday name.
Fix: Day of the week stays the same. Only Day number in week changes when you switch the week start.

Mistake: Reading Day of year as the day of the month.
Fix: Day of year counts from January 1 as day 1 through the whole year, not just within one month.

Mistake: Using Auto near 1582 and assuming every country changed calendars on the same day.
Fix: Use the Calendar note as a caution. If you are matching a specific historical source, test Julian and Gregorian directly.

• Auto switches at 1582-10-15, which matches the initial Gregorian reform date, but real adoption happened at different times in different countries.
• Dates from 1582-10-05 through 1582-10-14 need extra care in Auto mode because those dates were skipped in the first reform; the calculator should warn you and suggest checking Julian if needed.
• Leap year status depends on the selected Calendar system. The same year can be leap in Julian but not in Gregorian.
• Day number in week depends on Week starts on, but Day of the week does not.
• If your browser's date control cannot easily enter very early years, the math rules still stay the same, but the input method may limit what you can type.
• This calculator should reject impossible dates and year values below 1 instead of guessing.

How the calculator works

The calculator first reads the entered year, month, and day from Date. It then decides which calendar rules to use based on Calendar system. In Auto mode, dates on or after 1582-10-15 use Gregorian rules, and earlier dates use Julian rules.

Leap year rule

Leap years matter for Leap year status and Day of year.

Gregorian: leap = (year mod 4 = 0) and ((year mod 100 != 0) or (year mod 400 = 0))

Julian: leap = (year mod 4 = 0)

This means 1900 is not a leap year in Gregorian, but it is a leap year in Julian [1].

Convert the date to a day count

To find the weekday, the calculator converts the date into an integer day count called the Julian Day Number, or JDN. That gives one stable number for the date, and then the weekday comes from taking the remainder after dividing by 7.

a = floor((14 - month)/12)

y = year + 4800 - a

m = month + 12*a - 3

Gregorian JDN = day + floor((153*m + 2)/5) + 365*y + floor(y/4) - floor(y/100) + floor(y/400) - 32045

Julian JDN = day + floor((153*m + 2)/5) + 365*y + floor(y/4) - 32083

weekday_index = ((JDN + 1) mod 7)

The weekday index is then mapped to a name from Sunday through Saturday. This kind of modulo-7 day counting is a standard way to compute weekdays from dates [2].

Day number in week

After the weekday name is found, the calculator assigns Day number in week from 1 to 7. If Week starts on is Sunday, Sunday is 1. If Week starts on is Monday, Monday is 1. Only this number changes when the week-start option changes.

Day of year

Day of year is counted by adding the days in the earlier months of that year, then adding the day of the month. If the date is after February in a leap year, add 1 more day.

day_of_year = cumulative_days_before_month + day + leap_adjustment

So January 1 is always day 1, and December 31 is day 365 in a common year or 366 in a leap year.

Mini example

Example: use Gregorian mode for 2024-02-29. The year 2024 is a leap year because it is divisible by 4 and not blocked by the century rule. February 29 is valid, and its Day of year is 60. The weekday result for that date is Thursday, so with Sunday as the week start the Day number in week is 5.

Assumptions and limits

The calculator assumes year values start at 1 and rejects impossible dates instead of guessing. Historical results can differ from local records because countries adopted the Gregorian calendar at different times, even though Auto uses 1582-10-15 as one clear switch date [1].