Day of the Week Calculator
Finding out what day of the week a date falls on is useful in many situations: confirming what day a historical event occurred, checking what day your birthday lands on in any given year, planning around holidays or recurring events, and completing genealogical or legal research that requires knowing the weekday for a specific calendar date. The Gregorian calendar has been the international civil standard since the 16th century, and the relationship between dates and weekdays follows a predictable 400-year cycle. This calculator uses JavaScript's built-in date engine to determine the day of the week for any date you enter. It also shows the ordinal day of the year (1 through 365 or 366), the ISO 8601 week number (1 through 52 or 53), and the calendar quarter the date falls in. The ISO week number is particularly useful for business planning and scheduling, since ISO 8601 weeks always start on Monday and week 1 is defined as the week containing the first Thursday of the year. Enter any date from the Gregorian calendar to get instant results for all four outputs.
How the day of the week is calculated
The calculator uses JavaScript's Intl.DateTimeFormat API with the weekday: "long" option to return the full English day name. The ordinal day of the year is computed by subtracting January 1 of that year from the given date and dividing by the number of milliseconds in a day. The ISO week number uses the standard algorithm anchored to the first Thursday of the year.
Day of year: Math.floor((d - Jan1) / 86400000)
ISO week: Math.floor((d - startOfWeek1) / 604800000) + 1
Quarter: Math.ceil((month + 1) / 3) where month is 0-indexed
Worked example
For July 4, 2026:
- Day of week: Saturday
- Day of year: 185 (31 Jan + 28 Feb + 31 Mar + 30 Apr + 31 May + 30 Jun + 4 Jul = 185)
- ISO week: 27
- Quarter: Q3 (July falls in Q3)
Day of the week calculator: frequently asked questions
What day of the week is a given date?
Enter the date into the calculator above and the day name will display instantly. If you need a manual method, Zeller's congruence is a mathematical formula that derives the day of the week from the day, month, and year using modular arithmetic. For most everyday purposes the calculator is the fastest and most reliable option.
What is the ISO week number?
ISO 8601 defines a week numbering system where weeks run Monday to Sunday. Week 1 of a year is the week that contains the year's first Thursday. This means January 1 can fall in week 52 or 53 of the previous year, and late December dates can fall in week 1 of the following year. The calculator shows the ISO week number for any date.
What day of the week was January 1, 2000?
January 1, 2000 was a Saturday. You can verify this by entering 2000-01-01 into the calculator above. The year 2000 was notable for being a leap year (divisible by 400) and for the Y2K concerns about computer date handling.
How do I find the day number of the year?
The day number (also called the ordinal date) counts days from the start of the year. January 1 is day 1, January 31 is day 31, February 1 is day 32, and so on. December 31 is day 365 in a regular year and day 366 in a leap year. The calculator shows this for any date you enter.
What quarter is a given month in?
For the standard calendar year: Q1 covers January through March, Q2 covers April through June, Q3 covers July through September, and Q4 covers October through December. Note that fiscal quarters may differ by organisation; some companies start their fiscal year in a month other than January.
Official sources
- ISO 8601 date and time format: International Organization for Standardization.
- Astronomical applications and calendar resources: US Naval Observatory.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.