Find what day of the week any date falls on — past, present, or future. See the weekday name, ISO week number, and day of year.
Every calendar date falls on one of seven weekdays: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday. Finding the weekday for an arbitrary date used to require memorizing the Doomsday algorithm or a perpetual calendar; modern code returns it instantly with the standard date functions. A day-of-week calculator works for any date from year 1 to year 9999 and also returns the ISO week number, the day of year, and how many days until that date from today.
Today is Friday, August 21, 2026. Type any date from year 1 to year 9999.
Day of the Week
Friday
Friday, August 21, 2026
Today
| Weekday number | 5 (Monday = 1 ... Sunday = 7) |
| US convention (Sun=0) | Fri = 5 |
| ISO week number | Week 34 of 2026 |
| Day of year | Day 233 of 365 (64% through the year) |
| Next same weekday | In 7 days |
JavaScript’s Date.getDay() returns a number from 0 to 6, where 0 is Sunday and 6 is Saturday. The ISO 8601 standard starts the week on Monday, so we add 7 when the result is 0 to convert Sunday to 7.
The ISO week number follows the rule that week 1 is the week containing the first Thursday of the year. This means Jan 1 can belong to week 52 or 53 of the previous year, and Dec 31 can belong to week 1 of the next year — our calculator handles all of those edge cases.
Cross-checked against authoritative formulas (IRS, NIST, ACM) and updated for 2025/2026 tax years and rates.
No server roundtrip. All math runs in your browser so results appear the moment you finish typing.
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Each calculator includes the formula, plain-English explanation, and FAQs so you understand the result.
Estimates only. Calculator results are for educational and planning purposes — not professional advice. For binding decisions on taxes, investments, health, or legal matters, consult a qualified professional.
FAQ
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A day of the week calculator is a free tool that finds what day of the week any date falls on — past, present, or future. Our day of the week calculator also returns the ISO week number, day of year, days until that date, and the next time the same weekday occurs. The day of the week calculator works for any date from year 1 to year 9999.
How it works
Our day of the week calculator tells you what day any date falls on — past, present, or future — along with the ISO week number and day of year.
Use the date input to choose any calendar date. The day of the week calculator defaults to today, but you can pick any date from year 1 to year 9999.
The Today, +1 week, +1 month, year-end, and fixed-date buttons jump to commonly-tested dates without manual typing.
The big result shows the day of the week (Monday, Tuesday, etc.) and the full formatted date. The day of the week calculator also shows days until that date from today.
Below the main result, the table shows the weekday number (Mon=1 ... Sun=7), ISO week number, day of year, and the next time the same weekday occurs. Useful for project planning and historical lookups.