Question 1

How do I find what day of the week I was born?

Accepted Answer

Pick your date of birth in the date input. The tool returns the weekday name (Monday–Sunday) plus the ISO week number, US week number, day of the year, and whether you were born in a leap year. Calendar arithmetic uses the proleptic Gregorian calendar — the same one we use today, projected backward.

Question 2

What is the difference between ISO and US week numbering?

Accepted Answer

ISO 8601 weeks start on Monday and the week containing the first Thursday of January is week 1. The US convention starts weeks on Sunday and assigns Jan 1 to week 1 regardless. The two systems can disagree by a week at the start of January, which is why many spreadsheets explicitly label them.

Question 3

What day of the week was January 1, 2000?

Accepted Answer

Saturday. You can verify any historical date with this tool — pick the date and read the weekday. The Gregorian calendar reform happened on October 4–15, 1582 in Catholic countries; protestant and orthodox countries adopted it later, so dates before adoption may differ depending on which calendar a record used.

Question 4

How is a leap year determined?

Accepted Answer

A year is a leap year if it is divisible by 4, except century years (divisible by 100) which must also be divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be. Leap years insert Feb 29 to keep the calendar aligned with Earth's orbit (365.2425 days/year).

Question 5

What is the Doomsday algorithm?

Accepted Answer

A mental-math shortcut by John Conway for computing weekdays without a calendar. It picks an anchor weekday (the 'doomsday') for each year — for 2000s it's Tuesday — and uses easy-to-remember dates like 4/4, 6/6, 8/8, 10/10, 12/12, the last day of February, and 7/11 + 11/7 + 9/5 + 5/9. Useful for parlor tricks; this calculator is faster.

Question 6

Can I find the day of the week for ancient dates?

Accepted Answer

Yes, but be aware that historical dates are often given in the Julian calendar (used by the Roman Empire and most of Europe before 1582). This calculator uses the proleptic Gregorian calendar, so a Julian date of October 4, 1582 is shown as a different weekday than its Gregorian equivalent (October 14, 1582). For historical research, convert the date system first.

Day of Week Calculator

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