Day of the Week Calculator
Find out what day of the week you were born, and what day you turned or will turn 18, 30 and 50.
How to use the calculator
- Choose the birth date.
- The calculator automatically shows what day of the week the date fell on.
- Also see what day of the week the 18th and 50th birthday falls or fell on.
How the weekday is calculated
The calendar follows a fixed, predictable pattern: a standard Gregorian calendar repeats its weekday pattern at regular intervals, adjusted for leap years every four years (with a few exceptions at century marks). This makes it possible to calculate exactly what day of the week any date falls on, without counting manually.
The calculator uses the same method to find the weekday for the date you turned or will turn 18 and 50 — it simply creates a new date with the same month and day, but with the birth year plus 18 or 50, and finds the weekday for that date.
The calendar repeats — but not quite regularly
An ordinary year is 365 days, which is 52 weeks plus one day. A given date therefore shifts one weekday forward from year to year — but two days across a leap year. This pattern means the whole calendar repeats at regular intervals, typically after 6, 11 or 28 years, depending on where in the leap year cycle the date falls.
The Gregorian calendar only repeats completely after 400 years, because the leap year rule has exceptions for century years not divisible by 400. Within such a 400-year period, 1 January is also not evenly distributed across the weekdays — some weekdays occur slightly more often than others. A curiosity that follows: the 13th of the month falls on a Friday marginally more often than on any other weekday.
Frequently asked questions
Why is it possible to calculate the weekday for a date decades in the future or past?
The Gregorian calendar follows a fixed mathematical pattern with 365 days in common years and 366 in leap years (every four years, with a few exceptions at century marks). This pattern means the weekday for any date can be calculated precisely, no matter how far forward or back in time it is.
Does this calculator work for dates before the calendar reform (before 1582)?
No. The calculator assumes today's Gregorian calendar, which was introduced in 1582 and later adopted in many countries. It's not intended for historical dates from before the switch to the Gregorian calendar.
Can I use this to find the weekday for occasions other than a birthday?
Yes. The calculator finds the weekday for any date you enter, so you can use it for anniversaries, jubilees, or any other date you're curious about.