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Day of the Week Calculator

Returns the weekday of any Gregorian date together with its day-of-year number, the ISO 8601 week number and week-based year, and whether it falls on a weekend.

When to use

You need to know what weekday a date falls on, its ISO week number, or its ordinal day in the year.

Do not use when: You need to count days between dates (use date-difference) or determine leap years (use leap-year).

Formula

day_of_year = date − 1 January + 1; ISO week: take the Thursday of the same Monday-based week, iso_week = floor((day_of_year(Thursday) − 1) / 7) + 1 and iso_week_year = year of that Thursday

Inputs

Parameter	Type	Unit	Required	Description
`date`	date		default today	Any date in the proleptic Gregorian calendar (YYYY-MM-DD); defaults to the current UTC date.

Outputs

Output	Type	Unit	Description
`weekday`	string		Day of the week.
`day_of_year`	integer		Ordinal day number, 1 January = 1.
`days_remaining_in_year`	integer	days	Days after this date until 31 December.
`iso_week`	integer		ISO 8601 week number (1–53); weeks start on Monday and week 1 contains the year's first Thursday.
`iso_week_year`	integer		Year the ISO week belongs to (can differ from the calendar year around New Year).
`iso_week_date`	string		Date in ISO week-date notation YYYY-Www-D (D: 1 = Monday … 7 = Sunday).
`is_weekend`	boolean		true for Saturday or Sunday.
`days_in_year`	integer		365 or 366.

Example

2026-09-23: {"date":"2026-09-23"} → {"weekday":"Wednesday","day_of_year":266,"days_remaining_in_year":99,"iso_week":39,"iso_week_year":2026,"iso_week_date":"2026-W39-3","is_weekend":false,"days_in_year":365}

2021-01-01 (belongs to ISO week 53 of 2020): {"date":"2021-01-01"} → {"weekday":"Friday","day_of_year":1,"days_remaining_in_year":364,"iso_week":53,"iso_week_year":2020,"iso_week_date":"2020-W53-5","is_weekend":false}

GET https://tttkmbb.com/api/v1/calculate/day-of-week?date=2026-09-23

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/day-of-week (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/day-of-week · Markdown: https://tttkmbb.com/everyday/day-of-week.md · JSON definition: https://tttkmbb.com/everyday/day-of-week.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="day-of-week"
OpenAPI operationId: calculate_day_of_week
Freshness: daily. Authentication: none. Rate limit: fair use (see rate limits).

Sources

ISO week date (Wikipedia) (reference)
ISO 8601 – Date and time format (Wikipedia) (reference)

FAQ

Why can the ISO week year differ from the calendar year?

ISO weeks run Monday–Sunday and week 1 is the week containing the first Thursday of January, so 29–31 December can belong to week 1 of the next year and 1–3 January to week 52/53 of the previous year.

Are US-style week numbers (weeks starting Sunday) supported?

No. Only the ISO 8601 definition is returned; US calendars that start weeks on Sunday and call 1 January week 1 can differ by one.

Related calculators

Date Difference Calculator — Days and business days between two dates.
Leap Year Calculator — Check whether the year has 366 days.