Day of the Week Calculator
Pick any date and instantly see which weekday it lands on, along with its day-of-year and week number.
Example
Enter 2026-07-04: the calculator shows Saturday (a weekend), day-of-year 185, ISO week W27, with 180 days left in the year.
How it works
The browser parses the date and reads its weekday directly. Day-of-year counts days since January 1 (1-based), and the ISO 8601 week number is derived by finding the Thursday of that week.
Good to know
The Day of the Week Calculator takes any calendar date and tells you which weekday it falls on, then adds four extra details about that date: its day-of-year number, its ISO 8601 week number, how many days remain in the year, and whether the year is a leap year. It is handy for anyone planning around a specific date, settling a "what day was that?" debate, or checking a historical or future date without flipping through a calendar.
Reach for it when you need quick answers like whether a deadline lands on a weekend, what week number a project milestone falls in for an ISO-based reporting calendar, or which weekday a birthday, anniversary, or contract date will be in a given year. Because it accepts both past and future dates, it works equally well for looking back at a date that already happened and for scheduling something years ahead.
Reading the result is straightforward, but a few fields deserve attention. The weekday badge marks Saturday and Sunday as "Weekend" and everything else as "Weekday." The day-of-year counts January 1 as day 1, so the "days left in year" figure is simply the year's total length minus that count. The ISO week can be surprising near year boundaries: a date in early January or late December may carry a week number that belongs to the neighboring year, because ISO week 1 is defined as the week containing that year's first Thursday.
One practical caveat: the tool relies on the proleptic Gregorian calendar that browsers use, so weekdays it reports for very old dates (before the Gregorian reform of 1582) will not match what a historical Julian-calendar record would have shown. For modern dates it is exact, and since everything runs locally you can use it offline and on private dates without anything being sent anywhere.
Frequently asked questions
How is the day-of-year number calculated?
It counts the number of days from January 1 of that year up to and including your chosen date, so January 1 is day 1 and December 31 is day 365 (or 366 in a leap year).
Which week numbering does the ISO week use?
It uses the ISO 8601 standard, where weeks start on Monday and week 1 is the week containing the year's first Thursday. This is why early-January or late-December dates can belong to a neighboring year's week.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
How do I find out what day of the week I was born on?
Enter your full birth date into the calculator and it will display the weekday instantly, along with the day-of-year and week number for that date. Browser date math follows the Gregorian calendar, so results are accurate for any modern birth date.
Is January 1 always the first day of the week?
No. January 1 is always day 1 of the year, but it can fall on any weekday, and it is not necessarily in ISO week 1. ISO week 1 is the week containing the year's first Thursday, so a January 1 that lands on a Friday, Saturday, or Sunday belongs to the last week of the previous year.
What is the difference between day-of-year and week number?
Day-of-year is a single running count from 1 to 365 or 366 that tells you how many days into the year a date is. Week number groups days into weeks (1 to about 52 or 53); under the ISO 8601 standard those weeks start on Monday.
Can two dates have the same ISO week number in different years?
Yes. Week numbers reset each ISO week-year, so for example W27 exists in essentially every year, just covering different calendar dates. To identify a week uniquely you need both the week number and the year.
Why does the calculator say a December date is in week 1?
Under ISO 8601, the final days of December can belong to week 1 of the following year if they fall in the week that contains the next year's first Thursday. This is normal behavior, not an error.
How many days are in a leap year and how often do they occur?
A leap year has 366 days instead of 365, with February 29 added. Leap years occur every 4 years, except century years that are not divisible by 400 (so 1900 was not a leap year, but 2000 was).
Does the day of the week repeat on a regular cycle?
For a fixed date, the weekday repeats on a 28-year cycle in the Gregorian calendar in many cases, though leap-year rules at century boundaries can shift the pattern. The simplest way to know the exact weekday for a given year is to enter that specific date.
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