Day of the Week Calculator
Pick any date to find which day of the week it falls on, whether it is a past birthday, a future deadline, or a historical event. The calculator also reports the day of the year and the ISO 8601 week number. It uses the proleptic Gregorian calendar, so the modern rules apply consistently across all dates.
How the weekday is found
Weekday index = date's day-of-week on the Gregorian calendar
0 = Sunday, 1 = Monday, ... 6 = Saturday
Day of year = days since 1 January + 1
ISO week = week containing the Thursday of that week
The arithmetic is the same the world's calendar systems use: a fixed cycle of seven weekdays anchored to the Gregorian leap-year rule (every 4 years, except centuries not divisible by 400).
Worked example
4 July 1776 falls on a Thursday under the proleptic Gregorian calendar. It is the 186th day of that leap year and lies in ISO week 27.
Day of the week: frequently asked questions
How does the calculator find the day of the week?
It treats the date on the proleptic Gregorian calendar and computes the weekday directly from the calendar arithmetic the browser's date engine uses. The same rule that gives today's weekday extends backward and forward across centuries.
Does it work for historical dates?
It uses the proleptic Gregorian calendar, meaning the modern Gregorian rules are applied to all dates, including those before the calendar was adopted in 1582. Historical records that used the older Julian calendar may show a different weekday, so treat very old dates with care.
What is the day of the year?
The day of the year is the count of days from 1 January, so 1 January is day 1 and 31 December is day 365 (or 366 in a leap year). It is useful for ordinal date references and scheduling.
What is the ISO week number?
The ISO 8601 week number assigns each week a number from 1 to 53. Week 1 is the week containing the first Thursday of the year, and weeks run Monday to Sunday. This is the standard used across Europe and in many business systems.
Official sources
- U.S. Naval Observatory, astronomical applications: aa.usno.navy.mil (calendar reference).
- ISO 8601 week-numbering standard.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.