Day of the Week Calculator

Find the weekday for any date

Select a past or future date and the calculator shows its day of the week, ISO day number, and whether it falls on a weekday or weekend. Everything is calculated in your browser.

ISO day numbers begin with Monday as 1 and end with Sunday as 7. Naming the convention matters because software libraries and calendars do not all number Sunday the same way.

Why a weekday can be calculated

Every ordinary year moves the same calendar date forward by one weekday because 365 leaves a remainder of one when divided by seven. A leap year moves dates after February forward by two. Following those shifts forward or backward determines the weekday for any valid date.

The leap-year rule has one important exception. Years divisible by 4 are normally leap years, century years are not, and century years divisible by 400 are restored. That makes 2000 a leap year but 1900 and 2100 ordinary years. The calculator applies the complete rule.

A calendar date has no time zone

The tool treats the selection as a calendar date rather than a moment at midnight in your local time zone. Internally it uses UTC date arithmetic, where every day is exactly the same length.

This prevents a common daylight-saving bug. Around a clock change, two local midnights can be only 23 hours or as many as 25 hours apart. Code that divides that duration by 24 hours can lose or gain a day. UTC calendar arithmetic has no such transition, so the weekday remains stable wherever the page is opened.

Historical dates and the Gregorian calendar

The calculation uses the Gregorian calendar and extends its rules backward, a convention known as the proleptic Gregorian calendar. That makes results consistent and is the convention used by many computer date systems.

It does not mean every old document used the same calendar. Countries adopted the Gregorian calendar at different times and often skipped dates during the change from the Julian calendar. A date copied from a historical local record may therefore need that record’s original calendar and location before its weekday can be compared accurately.

Useful checks

A weekday lookup can verify the date of an appointment, identify the weekday of a birthday or anniversary, plan a future event, or check a date mentioned in a document. For spans between dates, use a date-difference calculator; for work schedules, use a business-days calculator that can account for weekends and holidays.

Frequently asked questions

How can I find what day of the week a date was?

Choose the calendar date. The calculator returns its weekday, ISO day number, and whether it is a weekday or weekend.

Can I check a future date?

Yes. The Gregorian calendar repeats according to fixed leap-year rules, so a future weekday can be calculated exactly without waiting for that date.

What are ISO day numbers?

ISO 8601 numbers Monday as 1, Tuesday as 2, through Sunday as 7. This avoids ambiguity between systems that number Sunday as either 0 or 1.

Does daylight saving affect the answer?

No. A calendar date has no clock time or time zone. The calculator uses UTC calendar arithmetic internally, so a 23-hour or 25-hour local day cannot shift the result.

How are leap years handled?

A year divisible by 4 is normally a leap year, but century years must also be divisible by 400. Thus 2000 was a leap year and 2100 will not be.

Will very old historical dates match records from that place?

Not always. The calculator extends today's Gregorian calendar backward. A historical country may have used the Julian calendar or adopted the Gregorian calendar on a different date, so contemporary records can use a different weekday or date label.

Is the date sent to a server?

No. The weekday calculation runs in your browser and needs no account or external calendar service.