How it works

A useful distinction

Annual dates move across weekdays

The same month and day usually moves one weekday forward the next year, or two when the interval includes February 29. Check each year separately when planning anniversaries instead of reusing last year’s weekday.

Finding a weekday for a calendar date

Enter the complete date, including the year, because the same month and day can fall on different weekdays in different years. The result uses the calendar convention supported by the tool. A weekday calculation identifies the calendar label only; it does not determine whether that date is a public holiday, school day, or working day in a particular location.

Day of the week: formula and assumptions

The Gregorian calendar and ISO week-date convention determine the result.

ISO weeks begin on Monday; week 1 contains January 4.

Worked example

Using the Day of the week example:

Inputs

Date: 2026-10-09

Method

The Gregorian calendar and ISO week-date convention determine the result.

Result

Example result

Weekday: Friday

Day of year: 282

ISO week: 2026-W41

These are illustrative inputs. Change the values to explore a different scenario.

Formula references

NIST: Guide to the SI (opens in a new tab)

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Why can an early-January date belong to the previous ISO year?

ISO week 1 is the week containing January 4. Dates near New Year can belong to a different ISO week-year.

Do ISO weeks begin on Sunday?

No. This calculator's ISO week convention starts each week on Monday.

Prepared by Finflo. See our methodology and editorial policy.

Day of the Week Calculator

Your inputs