{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"day-of-week"},"result":{"entity_type":"calculator","id":"day-of-week","calculator_id":"day-of-week","canonical_url":"https://tttkmbb.com/everyday/day-of-week","name":"Day of the Week Calculator","title":"Day of the Week Calculator – Weekday, Day of Year and ISO 8601 Week Number","category":"everyday","category_name":"Everyday Life","tool_name":"calculate_day_of_week","featured_mcp_tool":false,"description":"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.","use_when":"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).","inputs":[{"name":"date","label":"Date","type":"date","required":false,"default":"today","description":"Any date in the proleptic Gregorian calendar (YYYY-MM-DD); defaults to the current UTC date.","example":"2026-09-23"}],"outputs":[{"name":"weekday","label":"Weekday","type":"string","decimals":4,"description":"Day of the week."},{"name":"day_of_year","label":"Day of year","type":"integer","decimals":4,"description":"Ordinal day number, 1 January = 1."},{"name":"days_remaining_in_year","label":"Days remaining in year","type":"integer","unit":"days","decimals":4,"description":"Days after this date until 31 December."},{"name":"iso_week","label":"ISO week number","type":"integer","decimals":4,"description":"ISO 8601 week number (1–53); weeks start on Monday and week 1 contains the year's first Thursday."},{"name":"iso_week_year","label":"ISO week-based year","type":"integer","decimals":4,"description":"Year the ISO week belongs to (can differ from the calendar year around New Year)."},{"name":"iso_week_date","label":"ISO week date","type":"string","decimals":4,"description":"Date in ISO week-date notation YYYY-Www-D (D: 1 = Monday … 7 = Sunday)."},{"name":"is_weekend","label":"Weekend","type":"boolean","decimals":4,"description":"true for Saturday or Sunday."},{"name":"days_in_year","label":"Days in year","type":"integer","decimals":4,"description":"365 or 366."}],"input_schema":{"type":"object","properties":{"date":{"description":"Any date in the proleptic Gregorian calendar (YYYY-MM-DD); defaults to the current UTC date.","type":"string","format":"date","default":"today","examples":["2026-09-23"]}},"additionalProperties":false},"output_schema":{"type":"object","properties":{"weekday":{"description":"Day of the week.","type":"string"},"day_of_year":{"description":"Ordinal day number, 1 January = 1.","type":"integer"},"days_remaining_in_year":{"description":"Days after this date until 31 December. Unit: days.","type":"integer","x-unit":"days"},"iso_week":{"description":"ISO 8601 week number (1–53); weeks start on Monday and week 1 contains the year's first Thursday.","type":"integer"},"iso_week_year":{"description":"Year the ISO week belongs to (can differ from the calendar year around New Year).","type":"integer"},"iso_week_date":{"description":"Date in ISO week-date notation YYYY-Www-D (D: 1 = Monday … 7 = Sunday).","type":"string"},"is_weekend":{"description":"true for Saturday or Sunday.","type":"boolean"},"days_in_year":{"description":"365 or 366.","type":"integer"}}},"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","sources":[{"name":"ISO week date (Wikipedia)","url":"https://en.wikipedia.org/wiki/ISO_week_date","type":"reference","retrieved_at":"2026-09-23"},{"name":"ISO 8601 – Date and time format (Wikipedia)","url":"https://en.wikipedia.org/wiki/ISO_8601","type":"reference","retrieved_at":"2026-09-23"}],"freshness":{"type":"daily","max_age_seconds":86400,"note":"Depends on the current date unless the date is supplied."},"examples":[{"name":"2026-09-23","inputs":{"date":"2026-09-23"},"expected":{"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},"url":"https://tttkmbb.com/api/v1/calculate/day-of-week?date=2026-09-23"},{"name":"2021-01-01 (belongs to ISO week 53 of 2020)","inputs":{"date":"2021-01-01"},"expected":{"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},"url":"https://tttkmbb.com/api/v1/calculate/day-of-week?date=2021-01-01"}],"faq":[{"q":"Why can the ISO week year differ from the calendar year?","a":"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."},{"q":"Are US-style week numbers (weeks starting Sunday) supported?","a":"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."}],"tags":["day of the week","what day is it","iso week number","week number","day of year"],"related":[{"calculator_id":"date-difference","reason":"Days and business days between two dates."},{"calculator_id":"leap-year","reason":"Check whether the year has 366 days."}],"links":{"html":"https://tttkmbb.com/everyday/day-of-week","markdown":"https://tttkmbb.com/everyday/day-of-week.md","json":"https://tttkmbb.com/everyday/day-of-week.json","api":"https://tttkmbb.com/api/v1/calculate/day-of-week","schema":"https://tttkmbb.com/api/v1/calculators/day-of-week","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-23"},"timestamp":"2026-09-23T23:32:54Z","next_actions":[{"tool":"run_calculator","calculator_id":"day-of-week","reason":"Run Day of the Week Calculator with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/everyday/day-of-week.md"}}