Day Of Week Calculator

Some dates stick in your head and one question always follows them: what day of the week was that? A birth date, a wedding, a historical anniversary, or a deadline two years from now all raise the same problem, because no ordinary calendar view reaches far enough back or forward to answer it.

This calculator finds the weekday for any date from year 1 to 9999, and it also answers the follow-up question: what day will it be a given number of days from a chosen date? Every lookup also reports the day of the year, the ISO week number, and whether that year is a leap year.

Quick Answer

How to use the day of week calculator

Start with the mode selector. Weekday of a date is the plain lookup. What day is N days away adds a day count, positive to move forward or negative to move backwards, and returns the resulting date and its weekday. Enter the date either with the picker or in the three manual boxes. The manual boxes matter more than they look: they are how you reach years the picker handles awkwardly, and if any manual box is filled, the tool uses the manual date and asks you to complete all three.

The four famous date buttons load a date into the picker in one click, which makes them a fast honesty check on the tool itself. Anyone can verify that the Moon landing was a Sunday before trusting the calculator with their own birthday. If you are sorting out the dates written into a contract rather than a single day, the agreement date calculator works through agreement dates in more depth.

Read the result in layers. The large weekday is the headline. Below it sit the day of the year, the ISO week with the year that week belongs to, the leap year status, and, in weekday mode, how far the date sits from today. That last figure is counted live, so it changes every day, which is exactly what you want when the question is how long ago a birth date was.

Sakamoto's method, worked by hand

The weekday of a Gregorian date is fixed arithmetic, because the calendar repeats exactly every 400 years: 146,097 days, which is precisely 20,871 weeks. This calculator uses the compact formula published by Tomohiko Sakamoto. Take the year, add its leap adjustments, add a small month code from a fixed table, add the day, and divide by 7. The remainder is the weekday, counting 0 as Sunday.

Here is the same method worked by hand for Thursday, October 8, 2026, one of the offset examples used on this page:

The January and February adjustment is the elegant part. A leap day lands at the end of February, so for the first two months of a year the formula borrows the previous year. That folds leap days into the same one-line calculation instead of needing a special case, and it is why the month codes for January and February look one step lower than a naive count would suggest.

Worked example: July 4, 1776, and 100 days later

The calculator is pre-filled with July 4, 1776, the signing date of the US Declaration of Independence, and every figure in this table is what the tool returns for it.

The Y2K row deserves a second look. January 1, 2000 was the first day of its calendar year, yet it belongs to ISO week 52 of 1999, because ISO weeks are owned by the year that contains their Thursday. Tools that report a week number without its ISO year hide exactly this trap, and it is the kind of detail that matters when a work schedule or a school term is planned by week numbers. For a related workplace date problem, converting leave balances with the annual leave hours to days calculator is often the next step after pinning down which weekdays a break actually covers.

Why your birthday moves weekday every year

A common year has 365 days, which is 52 weeks plus 1 day, so every date shifts one weekday forward the following year. A leap year has 366 days, so a date after February shifts by two when a leap day falls in between. That simple pattern explains most birthday trivia: your birth date does not return to its original weekday on a tidy schedule, because the one and two day shifts mix irregularly across the four year leap cycle, and century years break the pattern again when they skip their leap day, as 1900 did and 2100 will.

It also explains why planning by weekday alone goes wrong. A monthly event set for the first Saturday will drift between the 1st and the 7th of the month, and an annual deadline set on a fixed date will land on a weekend roughly two years in seven. Checking the actual weekday for the specific year, rather than assuming last year's pattern repeats, is the whole job of this tool. When you need to subtract a span of time from a deadline instead, the deduct time calculator handles that direction of the problem.

Old dates, the Gregorian reform, and the ISO week

The Gregorian calendar was proclaimed in 1582 as a reform of the Julian calendar, according to Britannica's history of the Gregorian calendar, and countries adopted it at very different times: Catholic Europe in 1582, Britain and its colonies in 1752, others later still. Before a country's switch, its records were kept in Julian dates. This calculator applies today's Gregorian rules backwards across the whole range, the proleptic Gregorian convention used by ISO 8601 and by computer calendars. That keeps every date internally consistent, but for a pre-reform date it can differ from what a local almanac of the era would have printed.

The ISO week system has its own logic, set out in the ISO week date system: weeks start on Monday, week 1 is the week containing the year's first Thursday, and a year has 52 or 53 weeks. The practical consequence sits at the year boundary. A few late December days can belong to week 1 of the next ISO year, and a few early January days can belong to the last week of the previous one, which is why this calculator always reports the ISO year alongside the week number.

What competing calculators miss

The weekday tools reviewed for this page mostly get the weekday itself right, because the arithmetic is well known. The gaps sit around the answer.

Miss 1: silent acceptance of impossible dates. Several tools roll February 30 into March and report a weekday for a date that never existed. This calculator validates the day against the real length of that month in that year and explains the error, naming how many days that February actually has.

Miss 2: browser Date traps. Tools built directly on a browser's date object inherit its quirks, including the classic trap where years under 100 are read as 19xx unless the code takes special care, and midnight UTC parsing that can shift a weekday by time zone. Pure arithmetic has neither problem, which is how this tool covers year 1 to 9999 with the same code path.

Miss 3: a week number with no ISO year. As the Y2K example shows, that omission produces a wrong answer at exactly the year boundaries where week numbers get used for planning.

Miss 4: no offset mode. Knowing a date's weekday rarely ends the question. The practical follow-up is what day it will be 100 days later, or 90 days before a deadline, and most single-purpose tools send you to a separate date adder for it. Here it is a mode switch on the same tool, with the same validation.

Frequently Asked Questions

Add This Calculator to Your Website

References