How to use the day of the week calculator
- Pick a date. It starts on today's date in your browser.
- Read the weekday in the headline.
- Check the day of the year, the days left in the year, the ISO week and the quarter below it.
- Open the steps to see the leap-year explanation for that year.
Formula
Day number = days counted from a fixed starting date to your date
Weekday = day number modulo 7, mapped to Sunday through Saturday
Day of the year = days from January 1 to the date, counting January 1 as day 1
Days left = days in the year - day of the year
A leap year has 366 days. A year is a leap year when it is divisible by 4, except century years, which must also be divisible by 400.
Worked examples
October 10, 2026: January through September have 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 = 273 days, so October 10 is day 283 of 365. That leaves 82 days. January 1, 2026 is a Thursday, and 282 days later is 40 weeks and 2 days later, which is a Saturday.
February 29, 2024: January has 31 days, so February 29 is day 60 of 366, leaving 306 days. It was a Thursday.
Century years: 2000 was a leap year (divisible by 400) with 366 days. 1900 was not (divisible by 100 but not 400), so it had 365 days and January 1, 1900 was a Monday.
How the weekday is found
Weekdays repeat in a cycle of seven, so only the remainder after dividing the number of days by 7 matters. The calculator converts the date to a day count and reads the weekday from the remainder. It does not rely on your device's time zone, so the result is the same everywhere.
Because 365 is one more than 364, which is exactly 52 weeks, the weekday of a given date moves forward by one each normal year and by two after a leap day. That is why July 4 falls on a different weekday each year.
Week of the year and the quarter
The week shown is the ISO 8601 week, where weeks start on Monday and week 1 contains the first Thursday. For other week systems and for the dates inside a week, use the week number calculator.
Quarter 1 is January through March, quarter 2 is April through June, quarter 3 is July through September and quarter 4 is October through December.
Dates before 1582
The calculator extends today's Gregorian calendar backward. Many countries used the Julian calendar well after 1582, so a historical date recorded at the time may differ by several days from the result here.
Assumptions and limits
- Dates are plain calendar dates in the Gregorian calendar from the year 1000 to 9999.
- No time zone is involved, so the answer does not change with where you are.
- The week number shown follows ISO 8601, not the numbering used by some calendars and planners.
- Days left in the year does not count the date you entered.
Frequently asked questions
What day of the week was I born?
Enter your birth date. The headline gives the weekday, and the day of the year shows how far into the year it fell. For your exact age in years, months and days, use the age calculator.
How many days are left in the year?
The calculator subtracts the day of the year from 365, or from 366 in a leap year. Today does not count as a remaining day, so on December 31 the answer is 0.
How do I know if a year is a leap year?
It must be divisible by 4. Years divisible by 100 are not leap years unless they are also divisible by 400. So 2024 and 2000 are leap years, while 2100 and 1900 are not.
Why is February 29 on a different weekday every time?
It comes every four years, and four years is 1,461 days, which is 208 weeks and 5 days. So the weekday shifts by five each time, and the pattern repeats only every 28 years within a century.