What this calculator does
Which weekday a date falls on is the sort of thing that is obvious for next Tuesday and impossible for a date forty years out. This gives it for any date, along with the ISO week number, the day of the year and the calendar quarter.
The ISO week number is the part worth understanding, because it does not simply count weeks from 1 January. Week 1 is the week containing the first Thursday of the year, which means the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next.
The formula
The weekday comes directly from the date. The day of the year counts from 1 January, taking leap years into account, and the days remaining is the balance of the year from that point. The ISO week number is worked out from the rule that week 1 contains the first Thursday of the year, with weeks running Monday to Sunday.
| Term | Meaning |
|---|---|
| ISO week | A Monday-to-Sunday week numbered under ISO 8601, where week 1 is the one containing the first Thursday of the year. |
| Day of the year | The position of the date counting from 1 January, from 1 to 365 or 366. |
| Leap year | A year divisible by 4, except centuries not divisible by 400, giving 366 days. |
| Quarter | The three-month block the date falls in, from Q1 for January to March through to Q4. |
The inputs explained
| Field | What to enter |
|---|---|
| Date | The date to look up. Leave it on today or enter any past or future date. |
When to use it
Checking what day an event falls on
A date some way out gives no hint of its weekday, and whether a deadline or an anniversary lands on a weekend usually changes what happens around it.
Working with ISO week numbers
Manufacturing, retail planning and much European business scheduling run on week numbers rather than dates, and converting between the two is a frequent need.
Finding how much of the year is left
The day of the year and the days remaining are the figures behind any year-to-date measure or year-end countdown.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What weekday and week number does each date fall on?
Four dates through 2026 plus one leap day from 2024, for comparison.
| Date | Day of the week | ISO week number | Day of the year | Leap year? |
|---|---|---|---|---|
| 2026-01-01 | Thursday | 1 | 1 of 365 | No |
| 2026-03-15 | Sunday | 11 | 74 of 365 | No |
| 2026-07-01 | Wednesday | 27 | 182 of 365 | No |
| 2026-12-25 | Friday | 52 | 359 of 365 | No |
| 2024-02-29 | Thursday | 9 | 60 of 366 | Yes |
Questions
How is the ISO week number defined?
Week 1 is the week containing the first Thursday of the year, with weeks running Monday to Sunday. An equivalent statement is that week 1 is the one containing 4 January. This means a year has either 52 or 53 ISO weeks, and the first or last few days of a calendar year can belong to a week numbered in the adjacent year.
Why does my spreadsheet give a different week number?
Because several week-numbering conventions exist. Some count week 1 as the week containing 1 January, some start weeks on Sunday, and spreadsheet functions often default to one of those rather than to ISO. Where the number is being shared with others, say which convention you are using.
Which years are leap years?
Years divisible by 4, except centuries, which must also be divisible by 400. 2024 is a leap year and 2026 is not. 2000 was one, because it divides by 400, while 1900 was not. The rule keeps the calendar aligned with the solar year to within about a day in 3,000 years.
Can a year have 53 ISO weeks?
Yes. It happens when 1 January falls on a Thursday, or on a Wednesday in a leap year, which works out to roughly 71 years in every 400. Systems that assume 52 weeks break in those years, which is a recurring source of reporting errors.
For the number of days between two dates, see the days between dates calculator. For how many weeks remain in the current year, see the weeks left in the year calculator.