Day of the Week Finder
Find the day of the week, day of year and ISO week for any date.
Overview
The Day of the Week Finder answers "what day was 14 July 1789?" or "what day will my 50th birthday fall on?" in a single click. Enter any Gregorian date and you get the weekday name, the day-of-year ordinal (1–366), the ISO 8601 week number, and the Monday-start range of that week.
Useful for genealogists confirming the weekday of an ancestor's marriage, historians fact-checking a battle date, lottery and trivia enthusiasts settling bar bets, and anyone curious about what day of the week they were born on.
How it works
The calendar used is the proleptic Gregorian — that is, the modern leap-year rules are projected backwards through 1582 and earlier — which is the convention used by ISO 8601 and most programming languages. Weekdays are computed using a Zeller-style direct formula equivalent to the .NET DayOfWeek property.
ISO week numbers follow the standard rule that week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4), with weeks running Monday through Sunday. Day-of-year is a simple 1-based ordinal that ranges from 1 on January 1 to 365 or 366 on December 31 depending on whether the year is a leap year.
Examples
1789-07-14 → Tuesday, day 195, ISO week 1789-W29
2026-05-18 → Monday, day 138, ISO week 2026-W21
2032-02-29 → Sunday, day 60, ISO week 2032-W09
1969-07-20 → Sunday, day 201, ISO week 1969-W29
FAQ
Does it use the Julian or Gregorian calendar for old dates?
The proleptic Gregorian, so a 1500 date returned here will not match a historical document written under the Julian calendar in use at the time. Subtract ten days for a rough Julian conversion through 1582.
What range of years does it support?
Years 1 through 9999, matching the .NET DateTime type. For older dates use the Julian date converter.
Why does my January 1 sometimes show ISO week 53?
When January 1 is a Friday, Saturday, or Sunday it belongs to the last ISO week of the prior year. Week 1 begins on the following Monday, which contains the first Thursday of January.
Are there 366 days in any year?
Yes — leap years (divisible by 4, except centuries not divisible by 400) have 366 days. Day 60 in a leap year is February 29; in a common year it is March 1.
Can I get the Julian day number too?
For astronomical Julian dates (JD / MJD) use the dedicated Julian date converter — this tool focuses on the civil calendar.