Month and year shift
a = ⌊(14 − month) / 12⌋ ; y = year − a ; m = month + 12a − 2
Day of Week Calculator
Enter a date to see instantly whether it falls on a Monday, a Tuesday or another day, for any Gregorian date from year 1 to 9999.
Enter a valid date (day, month and a 4-digit year).
Day of the week
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How does this tool work?
What day of the week were you born? Does 25 December 2026 fall on a weekend? Is a meeting set for “the 14th of next month” a working day? This calculator gives the name of the day (Monday, Tuesday… Sunday) for any date in the Gregorian calendar, without leafing through a calendar or counting days by hand. The result appears as soon as the date is complete, together with the date written out in full so you can check that day and month were not swapped.
Weekdays follow a 7-day cycle that has never been interrupted since antiquity: even when the Gregorian calendar was introduced in 1582, ten dates were removed (5 to 14 October) but no weekday was skipped — Thursday 4 October was followed by Friday 15 October. Counting the days elapsed since a reference date and taking the remainder of the division by 7 is therefore enough.
The tool treats the date as a pure civil date in universal time, so the time zone of your device can never shift the result by a day. It uses the proleptic Gregorian calendar, meaning the Gregorian rule extended before 1582, as ISO 8601 does.
By hand, the congruence described by Claus Tøndering in his Calendar FAQ gives the same answer: treat January and February as months 13 and 14 of the previous year, then add the day, the year, the leap-year corrections and a month term. The remainder modulo 7 gives 0 for Sunday, 1 for Monday and so on.
How to use this tool
Enter the date
Type the date in month/day/year order or pick it in the calendar. The year has four digits (0050, 1789, 2026).
Read the weekday
The name of the day is shown in large type, followed by the full date so you can check your input.
Check the calendar of the time
For a date before the country adopted the Gregorian calendar, convert the Julian date to its Gregorian equivalent first.
Day of week formula (Gregorian calendar)
All divisions are integer divisions (keep the whole part). January and February count as months 13 and 14 of the previous year, which places 29 February at the end of the calculation year.
Day of the week
d = (day + y + ⌊y/4⌋ − ⌊y/100⌋ + ⌊y/400⌋ + ⌊31m/12⌋) mod 7
d = 0 for Sunday, 1 for Monday… 6 for Saturday.
- a
- 1 for January and February, 0 for the other months
- y
- calculation year (the previous year for January and February)
- m
- month rank counting March as 1 and February as 12
- d
- weekday, from 0 (Sunday) to 6 (Saturday)
Example for 9 October 2026: a = 0, y = 2026, m = 8; 9 + 2026 + 506 − 20 + 5 + 20 = 2546 and 2546 mod 7 = 5, a Friday.
Use cases
Birthdays and anniversaries
Find the day you were born, or the weekday of a future birthday, wedding anniversary or holiday.
Deadlines
Check whether a payment, notice or delivery deadline lands on a Saturday or Sunday before you announce it.
History and genealogy
Identify the weekday of a historical event or of a record in a parish register, taking into account the calendar used at the time.
Planning and travel
Put a departure, an event or a booking on the right weekday without misreading a wall calendar.
Examples
Storming of the Bastille
France had used the Gregorian calendar since 1582, so the result is the day people actually lived.
Input
07/14/1789Result
TuesdayChristmas 2026
Christmas Eve is Thursday 24 December, and New Year’s Day 2027 is also a Friday, one week later.
Input
12/25/2026Result
FridayA date before the British reform
Isaac Newton was born on 25 December 1642 in the Julian calendar still used in England, which is 4 January 1643 in the Gregorian calendar, a Sunday. Entering 12/25/1642 gives the weekday of the Gregorian 25 December (a Thursday), not of the date written in English records.
Input
12/25/1642Result
ThursdayTips and best practices
- A given date returns to the same weekday every 400 years exactly: the Gregorian cycle has 146,097 days, which is precisely 20,871 weeks.
- From one year to the next, a date moves forward by one weekday (two if a 29 February comes in between): 9 October 2026 is a Friday, 9 October 2027 will be a Saturday.
- For British dates before 14 September 1752, Russian dates before 14 February 1918 or Greek dates before 1 March 1923, the date in documents is usually Julian.
- Use the ISO week calculator for the week number, and the day of year calculator for the position of the date within its year.
Sources and references
- ISO — ISO 8601 date and time format (opens in a new tab)Page in English
- Claus Tøndering, Week-related questions (opens in a new tab), Calendar FAQPage in English
- Claus Tøndering, The Gregorian Calendar (opens in a new tab), Calendar FAQPage in English
- US Naval Observatory — Introduction to calendars (opens in a new tab)Page in English
- Unicode CLDR — Calendars, dates, weeks and locale conventions (opens in a new tab)Page in English
Frequently asked questions
How do I find out what day of the week I was born?
Enter your full date of birth and the tool shows the matching weekday. Because the date is handled without a time or time zone, the result is the same wherever you are today.
Does it work for very old dates?
Yes, from year 1 to year 9999, using the Gregorian rule extended into the past (the proleptic Gregorian calendar used by ISO 8601). Before 1582, and until the 20th century in some countries, documents used the Julian calendar, so the weekday shown is the one of the Gregorian date you entered.
Why does a historical weekday not match my sources?
Usually because the source gives a Julian date. The gap is 10 days between 1582 and 1700, then 11 days until 1800, 12 until 1900 and 13 since. Convert the date to the Gregorian calendar before entering it.
Which day is the first day of the week?
ISO 8601, used across most of Europe, starts the week on Monday (day 1) and ends it on Sunday (day 7). The United States, Canada and Japan usually show Sunday first. The weekday name does not change; only the calendar layout differs.
Is 29 February accepted?
Yes, in leap years only: years divisible by 4, except century years not divisible by 400. 29 February 2000 exists (a Tuesday); 29 February 1900 does not.
Are my dates sent to a server?
No. The calculation runs in your browser and the date never leaves your device.
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