{
    "name": "Day of the Week Calculator",
    "url": "https://calculators.geekmidst.com/calculators/day-of-the-week-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/day-of-the-week-calculator.md",
    "category": "Date, time & lifestyle",
    "description": "Find the day of the week for any date from year 1 to 9999, along with its day of the year, ISO week number and whether the year is a leap year.",
    "summary": "Enter a date to see which day of the week it falls on. Dates use the Gregorian calendar throughout, which is the calendar in civil use today.",
    "also_called": [
        "Weekday calculator",
        "What day was I born",
        "Day finder"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "date",
            "label": "Date",
            "type": "date",
            "default": "1969-07-20"
        }
    ],
    "outputs": [
        {
            "key": "weekday",
            "label": "Day of the week",
            "format": "text"
        },
        {
            "key": "long_date",
            "label": "Full date",
            "format": "text"
        },
        {
            "key": "day_of_year",
            "label": "Day of the year",
            "format": "integer"
        },
        {
            "key": "days_left",
            "label": "Days left in the year",
            "format": "integer"
        },
        {
            "key": "iso_week",
            "label": "ISO week",
            "format": "text"
        },
        {
            "key": "leap_year",
            "label": "Leap year",
            "format": "text"
        }
    ],
    "formula": {
        "text": "Count the days from a reference date of known weekday (1 January 1970 was a Thursday) and take the remainder after dividing by 7. ISO week = the week (Monday to Sunday) containing that week’s Thursday, numbered from the first week with a Thursday in the year.",
        "variables": {
            "Leap year": "divisible by 4, except century years not divisible by 400"
        }
    },
    "worked_example": {
        "inputs": {
            "date": "1969-07-20"
        },
        "steps": [
            "20 July 1969 is 165 days before 1 January 1970 (a Thursday).",
            "165 = 23 × 7 + 4, so the date falls 4 weekdays before Thursday: Sunday.",
            "Day of the year: 31 + 28 + 31 + 30 + 31 + 30 + 20 = 201."
        ],
        "result": "20 July 1969, the day of the Apollo 11 Moon landing, was a Sunday."
    },
    "interpretation": "ISO week numbers are used in business planning and in much of Europe; a few days at the start or end of a year can belong to the neighbouring year’s week 1 or week 52/53.",
    "assumptions": [
        "The proleptic Gregorian calendar is used for all dates."
    ],
    "limitations": [
        "Historical dates before a country adopted the Gregorian calendar (1582 in much of Catholic Europe, 1752 in Britain and its colonies) may have been recorded in the Julian calendar, with a different weekday mapping."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Why can 30 December be in week 1?",
            "answer": "ISO 8601 assigns each Monday-to-Sunday week to the year that contains its Thursday. 30 December 2024 was a Monday, and that week’s Thursday was 2 January 2025, so it is 2025-W01."
        },
        {
            "question": "Is 2000 a leap year?",
            "answer": "Yes. Century years are leap years only if divisible by 400, so 2000 was a leap year but 1900 was not."
        }
    ],
    "sources": [],
    "related": [
        {
            "name": "Date Difference Calculator",
            "url": "https://calculators.geekmidst.com/calculators/date-difference-calculator"
        },
        {
            "name": "Age Calculator",
            "url": "https://calculators.geekmidst.com/calculators/age-calculator"
        },
        {
            "name": "Time Duration Calculator",
            "url": "https://calculators.geekmidst.com/calculators/time-duration-calculator"
        },
        {
            "name": "Roman Numeral Converter",
            "url": "https://calculators.geekmidst.com/calculators/roman-numeral-converter"
        }
    ],
    "disclaimer": null,
    "publisher": "GM¹X CalcVerse"
}