Day of the Week Calculator: What Day Was It?
Choose a date to see the weekday it falls on, using the Gregorian calendar.
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Formula
Count the days between the date and a reference date, then weekday = (reference weekday + days) mod 7.
ISO week: the week (Monday to Sunday) belongs to the year of its Thursday; week = floor((day of year of that Thursday - 1) ÷ 7) + 1.
Worked example
29 February 2024: it is day 31 + 29 = 60 of the year. Counting from 1 January 2000 (a Saturday) the weekday works out to Thursday, in ISO week 9.
Frequently asked questions
How is the day of the week calculated?
The calculator counts the days from a known reference date and takes the remainder after dividing by 7. For example 1 January 2000 was a Saturday, so 7 days later, 8 January, was a Saturday as well.
What is the ISO week number?
ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. So 1 January 2021, a Friday, falls in week 53 of 2020.
What is the day of the year?
It is the position of the date within its year. 29 February 2024 is day 31 + 29 = 60, and 31 December 2024 is day 366 because 2024 is a leap year.
Does it work for dates in the distant past?
It applies the Gregorian calendar rules to every year, even before the calendar was adopted. 4 July 1776 comes out as a Thursday on that basis.