Day of the Week Calculator

DateMath counts the days from a fixed date to yours in the Gregorian calendar and takes the remainder after whole weeks, so every date in the years 1 to 9999 gets its weekday, leap days included. The same date is then numbered both ways: in the US, Sunday is day 1 of the week; in ISO 8601, Monday is day 1 and Sunday day 7. Today and tomorrow come from your device's date.

• The Gregorian calendar. DateMath treats every date as a Gregorian date, with its leap-year rule: a year divisible by 4 is a leap year, except century years not divisible by 400, so 2000 had a February 29 and 2100 will not. Source: Gregorian calendar.
• ISO 8601 week dates. DateMath numbers weeks by ISO 8601: weeks start on Monday, week 1 is the week with the year's first Thursday, and a year has 52 or 53 weeks. Source: ISO 8601.

DateMath's date math is tested against more than 100 cases worked out by hand. Every worked example and table on the site is computed with the same code the calculators run, so a page and its calculator can't disagree.

The first line of the calculator answers it: today's weekday and date, read from your device, with the day of the year and how many days are left. The line under it is tomorrow's weekday and date. Because the answer comes from your own clock, it changes at your midnight, and someone in another time zone can be on a different day for a few hours.

The same line answers “how many days so far this year”: day 1 is January 1, so on day 299 of a 365-day year, 298 days have passed and 66 are left after today. For any other date, pick it in the date field.

Sunday is the first day of the week on US calendars, and Monday is the first day under ISO 8601, the international standard for dates. Both are in everyday use, so “the 1st day of the week” and “the last day of the week” depend on which calendar you read:

• US calendars run from Sunday to Saturday: Sunday is day 1, Saturday the 7th and last day.
• ISO 8601 runs from Monday to Sunday: Monday is day 1, Sunday the 7th and last day. ISO week numbers, most of Europe and many business calendars use it.

The Sunday start is older. In the Jewish and Christian week the Sabbath, Saturday, is the seventh day, which makes Sunday the first: Hebrew still calls Sunday “yom rishon”, the first day, and Portuguese calls Monday “segunda-feira”, the second day. ISO 8601 starts the week on Monday so that the working week and the weekend stay in one numbered week.

Excel's WEEKDAY(date, 2) returns the ISO numbers instead, and Python's isoweekday() does too, while its weekday() counts Monday as 0. So is Saturday the last day of the week? On a US calendar, yes; in ISO 8601, Sunday is.

The weekday of the first day of every month, for planning a month ahead. Any other day of the month follows from it: the 8th, 15th, 22nd and 29th fall on the same weekday as the 1st.

Each of these dates has a page of its own, with the answer, the month's calendar and how to work the weekday out by hand:

DateMath counts days, but by hand the quickest exact method is Zeller's congruence. It adds up a few numbers taken from the date and keeps the remainder after dividing by 7. Take July 4, 2027:

1. Write the day (q = 4) and the month as a number from 3 for March to 14 for February (m = 7). January and February count as months 13 and 14 of the year before.
2. Split the year into the year of the century (K = 27) and the century (J = 20).
3. Add q + ⌊13 × (m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5 × J = 4 + 20 + 27 + 6 + 5 + 100 = 162.
4. Divide by 7 and keep the remainder: 162 = 7 × 23 + 1. The remainder counts from Saturday: 0 is Saturday, 1 Sunday, 2 Monday and so on to 6 for Friday.
5. 1 is Sunday, so July 4, 2027 is a Sunday.

A shortcut for nearby dates: a date moves one weekday later each year and two after a February 29, so if your birthday is a Friday this year, it is a Saturday next year unless a leap day comes first.

Enter your date of birth in the calculator and it shows the weekday, with the day of the year you were born on. To see your exact age, the total days you have lived and the weekday of your next birthday, use the age calculator, which also shows the weekday of birth. For a February 29 birthday, the leap year calculator counts the real birthdays.

In US English the weekday comes first, followed by a comma, then the month, the day and the year: Sunday, July 4, 2027. In the middle of a sentence a comma also follows the year. Short forms abbreviate both (Sun., Jul. 4, 2027), and ISO 8601 writes the date as 2027-07-04, usually without the weekday.

For more about the weeks themselves, the week number calculator gives the ISO and US week of any date, and the printable calendar shows whole months and years with their weekdays.