Weekday Calculator - Weekday Name and ISO Index

A weekday calculator turns any Gregorian date into the weekday name, a numeric weekday index, the ISO 8601 weekday number, the ISO 8601 week number, the day-of-year ordinal, and a leap-year flag in one screen. Type the date in YYYY-MM-DD order or leave the field blank to read today's weekday without retyping the current date.

• • Birthday and Anniversary Lookups: Find the weekday of a birth date or wedding anniversary so the milestone can be celebrated on the right day of the week.
• • Planning Around Future Events: Check whether a wedding, court date, release window, or surgery falls on a weekday or weekend before booking travel or staffing.
• • Historical and Genealogy Research: Verify the weekday of a recorded event from a family tree, a baptism record, or an old letter to align the calendar with the story.
• • Programming and Spreadsheet Formulas: Pull a 0-6 or 1-7 weekday index straight from the result panel into a JavaScript, Python, or spreadsheet helper without rewriting date math.

The calculator answers a stubborn question that calendars hide behind a wall of dates: given a single Y, M, and D, what day of the week is that? Knowing the answer turns the date into something you can plan around and reference in conversation with the same confidence as naming a month.

The result panel pairs the weekday name with the numeric weekday index, the ISO weekday number, the ISO week number, the day-of-year ordinal, and a leap-year flag, so the same lookup also covers 'what week of the year is that?' and 'is this a leap year?'.

When you also need the numbered day of the year that goes with the weekday name, Day Of The Year Calculator returns the same date's day-of-year ordinal and weekday on the same screen.

The calculator parses the entered date, applies the Gregorian leap-year rule, computes the day-of-year ordinal, derives the ISO weekday number from the standard Mon=1 numbering, and finally shifts the date to the Thursday of its week to count the ISO week number from the year's first Thursday.

• dateInput: Date string in YYYY-MM-DD order, or blank to use the user's local today converted to a UTC midnight.
• anchorWeekday: Weekday index of the chosen anchor (Sunday=0). The default anchor is 1970-01-01, a Thursday.
• totalDaysFromEpoch: Whole UTC days between the chosen anchor and the entered date, measured in 86,400-second steps.
• dayOfYear: Sum of the month lengths before the chosen month plus the day-of-month, with February extended to 29 days in leap years.
• firstThursday: Thursday of the ISO week that contains 4 January of the Thursday-shifted year, used as the ISO week-number origin.

The weekday name comes from the standard JavaScript Date.getUTCDay() lookup, which returns 0 for Sunday through 6 for Saturday and is mapped to plain English in the result panel.

The ISO 8601 weekday number is recomputed as ((weekdayIndex + 6) mod 7) + 1 so Monday becomes 1 and Sunday becomes 7.

According to International Organization for Standardization (ISO 8601), ISO 8601 numbers weekdays Monday=1 through Sunday=7 and identifies each week of the year using the week that contains the year's first Thursday.

Four small ideas decide what the calculator actually returns and why the same calendar date can land on a different day of the week from one year to the next.

These four ideas cover every output the calculator returns, so reading the result panel alongside the key concepts makes it easy to translate the weekday lookup into a scheduling or programming decision.

When the weekday matters because you are scheduling around a movable holiday, Easter Calculator returns the date and weekday of Easter for any year using the same Gregorian month-length rules.

Run a clean weekday lookup in four short steps. The result panel updates as you type, so you can compare dates and leap years without reloading.

1. 1 Type the date you want to look up: Enter a date in YYYY-MM-DD order, for example 2026-07-04, in the Date field. Leave the field blank to see today's weekday, ISO weekday, ISO week number, day of year, and leap-year flag.
2. 2 Read the weekday name and numeric index: Use the Weekday label for the plain-English day of the week and the Weekday Index (Sun=0) and ISO Weekday (Mon=1) rows to copy a numeric value into a spreadsheet or a programming helper.
3. 3 Check the ISO week number and day of year: Read ISO Week Number for the week used by international project plans and payroll cycles, and read Day of Year for the ordinal position in the calendar year.
4. 4 Confirm the leap-year flag: Read Leap Year to see whether the entered date falls in a common year (No) or a leap year (Yes), so you can verify February 29 inputs and trace the day-of-year shift for March 1 onward.

Leave the date field blank to see today's weekday alongside the same day's ISO week number, ISO weekday, day-of-year ordinal, and leap-year flag. Switch to 2024-02-29 to confirm the 2024 leap day falls on a Thursday in ISO week 9.

When the weekday lookup is the first step in a date-arithmetic workflow such as adding days or counting forward to a deadline, Date Calculator accepts the same YYYY-MM-DD input and returns the resulting date alongside its weekday and day-of-year.

The calculator replaces a mental calendar lookup, a programming library, and a leap-year edge-case check with one screen of results.

• • One Field, Six Verified Answers: Type a single date and read the weekday name, weekday index, ISO weekday, ISO week number, day of year, and leap-year flag in one place.
• • Live Today Mode: Leave the field blank to see today's weekday, ISO week number, and leap-year flag without typing the current date.
• • ISO 8601 Ready: The ISO weekday and ISO week number use the same Monday=1 and Thursday-anchored rules as ISO 8601, so the result panel lines up with European payroll cycles and ISO week-date calendars.
• • Leap-Year Safe: The Gregorian leap-year rule is applied first, so 29 February 2024 returns the correct 60th day of the year while 29 February 1900 and 29 February 2025 are rejected with a validation error.
• • Real-Time Recalculation: The result panel updates as the date changes, which makes it easy to compare a date in a common year against the same date in a leap year.

Most weekday lookups are quick everyday questions, so the calculator returns just the answers you need without the overhead of a full calendar view.

When the ISO weekday and ISO week number need to feed into a multi-week schedule or a recurring payroll cycle, Week Calculator applies the same Monday-anchored ISO 8601 rules across two dates and reports complete weeks plus leftover days.

The weekday formula is constant, but a handful of calendar rules change the answer by an entire weekday or trigger a validation error.

• • The calculator accepts Gregorian dates between 1900-01-01 and 2100-12-31. Dates outside this range, including most pre-Gregorian and far-future dates, are rejected because the leap-year rule and the ISO week algorithm assume the modern Gregorian calendar.
• • The result panel reflects the calendar date only; it does not include a local time zone, daylight-saving shifts, or a Julian day number. Use a dedicated time-zone calculator when the same date needs to be anchored to a wall-clock event.

According to U.S. Naval Observatory Astronomical Applications Department, a Gregorian year is a leap year if it is divisible by 4, except for years divisible by 100 that are not also divisible by 400.

According to Encyclopædia Britannica: Gregorian calendar, the Gregorian calendar assigns 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, and 31 days to January through December in a common year and extends February to 29 days in a leap year.

When the weekday matters because the date is also a seasonal milestone, Days Until Fall Calculator counts the days until the autumn equinox using the same Gregorian calendar arithmetic.