Day of Week Finder
The day of week finder tells you which weekday any date falls on. Enter any past or future date and instantly see the day name, ISO week number, and day of the year.
Perfect for planning events, checking historical dates, or satisfying your curiosity about what day of the week a particular date was or will be.
How it works
The weekday is determined using JavaScript's Date object. The ISO week number is calculated per ISO 8601: the week containing the year's first Thursday is week 1. Day of year = days elapsed since January 1.
Use cases
- Finding out what day of the week you were born
- Planning events on a specific weekday
- Checking historical dates and their weekdays
- Determining week numbers for project planning
Frequently asked questions
What day of the week was I born on?
Enter your date of birth into the finder and it instantly returns the weekday name, such as Monday or Friday. It works for any past or future date, so you can check births, anniversaries, or historical events in seconds.
How is the day of the week calculated from a date?
This tool uses JavaScript's Date object, which applies the proleptic Gregorian calendar to map any date to a weekday. Mathematically, the same result can be found with algorithms like Zeller's congruence, which computes the weekday from the day, month, and year.
What is an ISO week number?
The ISO 8601 week number labels each week of the year from 1 to 52 or 53. Week 1 is defined as the week containing the year's first Thursday, and weeks start on Monday. This standard is widely used in business, manufacturing, and project planning.
How do I find the day of the year for a date?
The day of the year is the count of days elapsed since January 1, so January 1 is day 1 and December 31 is day 365 (or 366 in a leap year). This finder shows that number automatically alongside the weekday and ISO week.
Does the calculator account for leap years?
Yes. Because it relies on the standard Gregorian calendar, February 29 is handled correctly and the day-of-year count reaches 366 in leap years. Leap years occur every year divisible by 4, except century years not divisible by 400.