Day of Week Calculator

Find the day of the week for any historical or future date, plus its day of year, ISO week number, and weekend status.

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Thursday

March

Weekday
Day of week Thursday (index 4)
Day of year 78
ISO week number Week 12
Month March
Weekend No

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Calendar Tool

Day of week calculator: find the weekday for any date

The day of week calculator tells you which day of the week — Monday through Sunday — any date in history or the future falls on. It also returns the day of the year (1–366), the ISO week number (1–53), and the quarter of the year. Useful for historical research, planning, and satisfying everyday curiosity about dates.

How the day of week is calculated

The most reliable method is Zeller's congruence or the Tomohiko Sakamoto algorithm, both of which use modular arithmetic to map any Gregorian calendar date to a day of the week without needing to consult a calendar. The formula accounts for the irregular lengths of months and the 400-year Gregorian leap year cycle.

The day of year is calculated by summing the days in each preceding month (using the correct February length for leap years) and adding the day of the month. The ISO week number follows the ISO 8601 standard: week 1 is the week containing the first Thursday of the year, and weeks run Monday to Sunday.

Day of week = (day + floor(2.6 × month − 0.2) + year + floor(year/4) + floor(century/4) − 2 × century) mod 7

Zeller's congruence (Gregorian); result 0=Sunday … 6=Saturday.

Day of year = Sum of days in Jan through (month−1) + day (+ 1 if after Feb in a leap year)

Returns a value from 1 (1 Jan) to 365 or 366 (31 Dec).

Practical uses

Historical researchers use the day of week calculator to verify or reconstruct timelines. If a diary entry says "Tuesday the 14th", confirming which month and year that date fell on a Tuesday narrows the date range considerably. Legal documents often specify weekdays ("the agreement shall be signed on a Monday"), and the calculator confirms which calendar dates satisfy that condition.

Event planners use it to check whether a chosen date falls on a weekend or to find the nearest weekday to a target date. HR teams use it to determine on which day of the week a hire date, termination date, or review date falls, since many employment law provisions depend on weekday versus weekend.

Frequently asked questions

What day of the week was I born on?

Enter your birth date into the calculator. It will return the full day name (for example, Wednesday) as well as the day of year and ISO week number for that date. Many people are surprised to discover their birth weekday.

 What is an ISO week number?

The ISO 8601 week number is a standardised way to identify a week in a year. Week 1 is defined as the week containing the first Thursday of the year (equivalently, the week containing 4 January). Weeks run Monday to Sunday. Some years have 53 ISO weeks.

 Does the calculator work for dates before the Gregorian calendar reform?

The calculator uses the proleptic Gregorian calendar for all dates, extending Gregorian rules backward in time. This does not match the Julian calendar used historically before the Gregorian reform (1582 in Catholic countries, later elsewhere). For historical research involving pre-reform dates, use a dedicated Julian calendar tool.

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Weekdays in Year Calculator

Count how many times each day of the week appears in a selected year, with totals for weekdays and weekend days.

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Days in Year Calculator

Find the exact number of days in any year — 365 for common years or 366 for leap years — with a clear leap-year note.

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Days in Month Calculator

Find how many days are in any month and year — accounting for leap-year rules for February.

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Leap Year Calculator

Check whether a given year is a leap year, see the days it contains, and find the nearest past and upcoming leap years.

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