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Day of the Week Calculator
Enter any date to find out which weekday it falls on. You also get the day of the year, the ISO 8601 week number, the quarter and whether the year is a leap year.
How the weekday is found
Computers store a date as a count of days from a fixed origin, and the weekday is the remainder when that count is divided by 7. This calculator reads the day count from the UTC date, so the answer does not depend on your time zone. Because the Gregorian calendar repeats exactly every 400 years, which is 20,871 weeks, weekdays follow a fixed pattern that can be worked out for any date.
As a sanity check, 1 January 1970 was a Thursday, and 29 February 2024 was also a Thursday.
ISO 8601 week numbers
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year, which is the same as the week containing 4 January. The consequence is that the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. For example, 1 January 2021 was a Friday, so it falls in week 53 of 2020.
The calculator finds the Thursday of the date's week, takes that Thursday's year as the ISO year, and counts weeks from the start of that year. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. Other week-numbering systems exist, for example ones that start weeks on Sunday, so a week number from a diary or payroll system may differ.
The leap year rule
A year is a leap year when it is divisible by 4, except century years, which must also be divisible by 400. So 2024 and 2000 are leap years, while 1900 and 2100 are not. This keeps the average calendar year at 365.2425 days, close to the solar year. Leap years have 366 days with the extra day on 29 February.
| Year | Divisible by 4 | Century rule | Leap year |
|---|---|---|---|
| 2024 | Yes | Not a century | Yes |
| 1900 | Yes | Not divisible by 400 | No |
| 2000 | Yes | Divisible by 400 | Yes |
| 2026 | No | Not applicable | No |
When this is useful
People use a weekday lookup for birthdays and anniversaries, for historic events, for planning which day a deadline falls on, and for payroll or reporting systems that use week numbers. To find a weekday after adding time, use the date calculator, which also shows the weekday of the result. For working-day counts, use the business days calculator.
Worked example
Which weekday was 1 January 2021, and which ISO week does it belong to?
Inputs
| Date | 2021-01-01 |
|---|
Result
1 January 2021
| Day of the year | 1 of 365 |
|---|---|
| Days left in the year | 364 |
| ISO 8601 week | Week 53 of 2020 |
| Quarter | Q1 |
| Leap year | No, 2021 is not a leap year |
| Days in this month | 31 |
Accuracy note: Results are calculated in your browser from the values you enter. Check anything that matters for legal, academic or financial purposes against the official source.
Frequently asked questions
How do I find the day of the week for any date?
Enter the date above and the weekday appears immediately. By hand, count days from a known reference date and take the remainder after dividing by 7.
What is an ISO week number?
It is the week number defined by ISO 8601, where weeks run Monday to Sunday and week 1 contains the first Thursday of the year. Some years have 53 such weeks.
Why does 1 January sometimes show as week 52 or 53?
Because ISO week 1 must contain the first Thursday, early January days can still belong to the last week of the previous year. The ISO year shown beside the week number tells you which year applies.
Is 1900 a leap year?
No. It is divisible by 4 but also a century year not divisible by 400, so February 1900 had 28 days. The year 2000 was a leap year.
Does my time zone change the answer?
No. The date you enter is treated as a calendar date, so the weekday is the same everywhere in the world for that date.