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    Weekday Calculator

    Find what day of the week a date falls on

    How It Works

    Overview

    The Weekday Calculator tells you which day of the week any calendar date falls on, along with the ISO week number, day-of-year ordinal, and calendar quarter. Whether you're verifying a contract date, looking up the day you were born, or scheduling a recurring meeting, this tool resolves any date from the year 100 forward in milliseconds.

    The underlying calculation uses the proleptic Gregorian calendar — the modern leap-year rule applied uniformly to all dates. Day-of-week is computed via JavaScript's Date.getDay(), which is mathematically equivalent to Zeller's congruence, the formula every computer-science student learns for hand-calculating weekdays.

    The Formula

    h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7

    Zeller's congruence (Gregorian variant) — where:

    • h = day of week (0 = Saturday, 1 = Sunday, … 6 = Friday)
    • q = day of the month
    • m = month, with January and February treated as months 13 and 14 of the previous year
    • K = year mod 100 (year within the century)
    • J = ⌊year / 100⌋ (zero-based century)

    ISO 8601 week numbering uses a different calculation: week 1 is the week containing the first Thursday of the year, equivalently the week containing January 4. Most years have 52 weeks; years where January 1 is a Thursday (or December 31 is a Thursday in a leap year) have 53.

    Worked Example

    Find the weekday for May 1, 2026:

    • q = 1, m = 5, year = 2026 → K = 26, J = 20
    • h = (1 + ⌊13(6)/5⌋ + 26 + ⌊26/4⌋ + ⌊20/4⌋ − 40) mod 7
    • h = (1 + 15 + 26 + 6 + 5 − 40) mod 7 = 13 mod 7 = 6
    • h = 6 → Friday

    Cross-check: May 1, 2026 is day 121 of the year, falls in ISO week 18, and belongs to Q2. The calculator returns all four values plus a weekend flag in a single click.

    When to Use This

    • Historical research. Find out which day of the week your great-grandparents were married or what day a famous battle was fought.
    • Contract verification. Confirm that a "first Monday of the quarter" clause actually maps to the date the other party listed.
    • Recurring meeting scheduling. Check what weekday a future board meeting falls on before sending invites.
    • Payroll cutoffs. See the ISO week number to align with European HR systems.
    • Birthday planning. Figure out years in advance which weekday a milestone birthday will land on so you can book a venue.
    • Crossword and trivia. Quickly verify "what day of the week was July 4, 1776?" — it was a Thursday.

    Common Mistakes to Avoid

    • Confusing ISO week with US week. Some US calendars number weeks starting from January 1 with Sunday as day 1, producing different week numbers than ISO 8601.
    • Forgetting that January 1–3 may belong to the previous ISO year. Treat the ISO year and calendar year as separate fields when filing reports.
    • Applying Gregorian rules to pre-1582 dates. Most of Europe used the Julian calendar before 1582; protocols and switchover dates varied by country into the 1900s in some regions.
    • Off-by-one in day-of-year. January 1 is day 1, not day 0. February 29 is day 60 in a leap year and doesn't exist otherwise.
    • Assuming weekends are universal. In several Middle Eastern countries the weekend falls on Friday and Saturday, not Saturday and Sunday.

    Frequently Asked Questions

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