Day of the week calculator

Choose a date to see whether it falls on a Monday, Tuesday or another day of the week.

Year

Day of the week

Monday

Read explanation below

How the day of the week calculator works

A calendar page connected to a seven-day weekday ring

Choose a day, month and year. The calculator checks that the date exists, applies the Gregorian calendar rules and returns the matching weekday.

Dates that make useful checks

Known dates are the quickest way to see what the result means and to catch a date entered in the wrong order.

For a birthday, appointment or anniversary, enter the printed civil date exactly as written. A weekday belongs to the date itself; no time of day is required.

The arithmetic behind the weekday

The calculation uses an integer form of the Gregorian calendar. January and February are treated as part of the preceding calculation year, then a month offset is added before division by seven.

w=(y+⌊y4⌋−⌊y100⌋+⌊y400⌋+tm+d) mod 7w=\left(y+\left\lfloor\frac y4\right\rfloor-\left\lfloor\frac y{100}\right\rfloor+\left\lfloor\frac y{400}\right\rfloor+t_m+d\right)\bmod 7

Here d is the day of the month, tm is the fixed offset for the selected month, and y is the adjusted year. The remainder w runs from 0 for Sunday through 6 for Saturday.

The divisions by 4, 100 and 400 encode leap years. Over 400 Gregorian years there are 146,097 days, exactly 20,871 weeks, so the weekday pattern repeats after 400 years.

A calendar answer can still need context

The calculator uses the proleptic Gregorian calendar for every year from 1 to 9999. “Proleptic” means that today’s Gregorian rules are extended backward even before the calendar was introduced. This gives one consistent mathematical answer, but an old local record may have used the Julian calendar or a regional changeover date.

A timezone does not change the weekday of a plain civil date. It matters only when a timestamp is converted between places and crosses midnight. If you start with an event time rather than a written date, convert it to the intended local date first.

Impossible dates are rejected. February has 29 days only in a Gregorian leap year, while April, June, September and November stop at day 30.

Questions people ask about weekday dates

The method is exact for its calendar, but the calendar choice and the date itself still matter.

Can the same date fall on a different weekday in another calendar?

Yes. A Julian calendar date and a Gregorian date with the same written numbers can refer to different civil days and therefore different weekdays.

Why is 1900 not a leap year but 2000 is?

Gregorian century years must be divisible by 400. The year 1900 fails that test, while 2000 passes it.

Does the weekday pattern repeat?

Yes. The Gregorian pattern repeats every 400 years because the full cycle contains an exact number of weeks.

Can I check a date before 1582?

Yes. The result uses the proleptic Gregorian calendar, which may differ from the calendar used by a historical document.

Do I need to enter a time or timezone?

No for an ordinary date. Add timezone handling only when you are converting a precise timestamp that may move across midnight.

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Calcopedia (en) → Day of the week