AllCalciHub logo AllCalciHub
Everyday

Day of Week Calculator

Find out what day of the week any date falls on. Also shows day of year, week number, and days until the next occurrence.

What is Day of Week Calculator?

The day of week calculator tells you exactly what day of the week any given date falls on, from ancient history to the far future. It also shows the day number of the year, the ISO week number, and how many days until the next time that weekday occurs.

How to use

  1. 1 Pick a date using the date picker, or type it directly.
  2. 2 Click Calculate to see the results.
  3. 3 The day of year shows how many days into the year the date falls.
  4. 4 The week number follows the ISO 8601 standard (week 1 contains the first Thursday of the year).
  5. 5 Days until next occurrence counts forward from today to the next time that same weekday falls.

Formula

Tomohiko Sakamoto algorithm: day = (y + floor(y/4) − floor(y/100) + floor(y/400) + t[m−1] + d) % 7, where t = [0,3,2,5,0,3,5,1,4,6,2,4] and y is decremented by 1 if m < 3.

Example calculation

July 4, 1776: y=1775 (since m<3 is false, y stays 1776), t[6]=5, d=4. Result: (1776+444−17+4+5+4)%7 = 2216%7 = 4 → Thursday.

Frequently asked questions

What is ISO week number?

The ISO 8601 week number counts weeks starting Monday. Week 1 is the week containing the first Thursday of the year. A year can have 52 or 53 weeks.

Is this calculator accurate for historical dates?

Yes, for dates on or after October 15, 1582, when the Gregorian calendar was adopted. Before that, the Julian calendar was used and results will differ from historical records.

What does 'day of year' mean?

Day of year (also called ordinal date) is how many days have passed since January 1. January 1 = day 1, December 31 = day 365 (or 366 in a leap year).

How is the next occurrence calculated?

Starting from today, the calculator counts forward to find the next date that falls on the same day of the week as your selected date.

What day was January 1, 2000?

January 1, 2000 was a Saturday. Enter the date in the calculator to verify.