Day of the Week Calculator
Enter any date, past or future, to instantly find which day of the week it falls on, its position in the year, and its ISO week number — all calculated in your browser.
The Day of the Week Calculator runs entirely in your browser. The date you enter never leaves your device and nothing is uploaded to ArrayKit.
Open the Date Calculator
About Day of the Week Calculator
The Day of the Week Calculator turns any calendar date into an instant answer: which weekday it lands on, whether that day is a weekend, how far into the year it falls, and which ISO-8601 week it belongs to. Type or pick a date decades in the past or centuries into the future — a birthday, an anniversary, a historical event, a delivery deadline — and it resolves the exact weekday using real calendar math, correctly accounting for leap years and month lengths rather than a rough estimate. It is handy for answering "what day was I born?", checking whether a due date lands on a weekend, or looking up the ISO week number a project milestone falls in. The whole calculation runs locally as you type; the date you enter never leaves your device.
Features
- Instant weekday for any date, past or future
- Accurate across leap years and every month length
- Day-of-year count (1 through 365 or 366)
- ISO-8601 week number, including the correct week-numbering year
- Clear weekend / weekday flag for scheduling checks
- Full written date (e.g. "Sunday, July 19, 2026") alongside the raw figures
- One-click copy of the weekday summary or any individual result
- Runs entirely in your browser with nothing uploaded
How to use the Day of the Week Calculator
- Pick or type a date in the date field, or click Today for the current date
- Read the weekday name and weekend/weekday badge at the top
- Check the day-of-year count to see how far into the year the date falls
- Check the ISO week number for scheduling or calendar-week references
- Copy the weekday summary or any single result with the copy button
Example
Input
2000-01-01
Output
Saturday
Common errors & troubleshooting
- The weekday looks off by one day compared to a date I remember. — Double-check the date was typed correctly (YYYY-MM-DD) — a common mix-up is swapping the month and day, which shifts the result to a different date entirely.
- The ISO week number seems to belong to the wrong year. — ISO-8601 weeks always run Monday to Sunday, so the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. The tool shows the correct week-numbering year alongside the week number for exactly this reason.
- Nothing appears after typing a date. — Make sure the date is a real calendar day — February 31 or April 31, for example, do not exist and are rejected until corrected.
Frequently asked questions
- How does the Day of the Week Calculator find the weekday for a date?
- It uses the date's calendar position with real leap-year and month-length rules to resolve the exact weekday, so results are accurate for dates centuries in the past or future, not just the current century.
- Can I use this weekday calculator to check what day I was born?
- Yes. Enter your date of birth and the tool immediately shows the weekday it fell on, along with its day-of-year and ISO week number for that year.
- What does the ISO week number shown by this tool mean?
- It is the ISO-8601 week number (1 through 52 or 53), where weeks run Monday through Sunday and week 1 is the week containing the year's first Thursday. The tool also shows the matching week-numbering year, since dates near January 1 or December 31 can belong to a different ISO year than the calendar year.
- Does the weekend flag account for different regional weekends?
- The weekend flag marks Saturday and Sunday, the standard weekend in most Western calendars. If your region uses a different weekend, use the weekday name directly rather than the weekend/weekday badge.
- Is the day-of-year figure adjusted for leap years?
- Yes. The day-of-year count and the 365/366 total shown alongside it both reflect whether the entered date's year is a leap year, so February 29 and every day after it are counted correctly.
- Does the Day of the Week Calculator send my date anywhere?
- No. The calculation runs entirely in your browser. The date you enter is used only on your device and is never uploaded to ArrayKit or any server.
Related tools
- Date Calculator — Add or subtract years, months, weeks and days from a date to find the resulting date.
- Date Duration Calculator — Calculate the duration between two dates in years, months, weeks, days and business days.
- Business Days Calculator — Add or subtract working days from a date, or count business days between two dates.
- ISO Week Number Calculator — Find the ISO-8601 week number for any date, and the date range for a given week.
- Age Calculator — Calculate exact age in years, months and days as of any date, plus totals in weeks, days, hours and next birthday.
- Time Calculator — Add or subtract times and durations, and sum multiple time spans.
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