Doomsday Algorithm Weekday Calculator

How it is calculated

Enter a date as YYYY-MM-DD (year 1 to 9999). The tool returns the weekday of that date, the weekday of the year's doomsday, and the day-of-year number. The doomsday is a weekday shared by easy anchor dates in the same year (4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 and the last day of February). The tool shows it as the weekday of 4 April. By hand, you take the anchor in the target month, then find (day − anchor day) mod 7 and move that many weekdays forward from the doomsday. The tool reaches the same answer directly from the calendar. Leap years follow the Gregorian rule, applied to every year, including those before the calendar was adopted anywhere. Non-existent dates (2023-02-29), year 0 and other layouts return no result.

Common mistakes

• Mixing up the year's doomsday with the weekday of the date you typed. They are the same only when your date happens to be an anchor day such as 4 April.
• Using the wrong February anchor by hand: it is the last day of the month, 28 February in common years but 29 February in leap years. 2024-02-29 is a Thursday, the same as 4 April 2024.
• Typing the date as DD.MM.YYYY or MM/DD/YYYY. The tool reads only year-month-day with hyphens, and rejects impossible dates such as 2023-02-29.
• Expecting a historical date to match the calendar of its time. Every date is treated as Gregorian, so a day before a country switched from the Julian calendar may differ from the weekday recorded then.
• Counting the offset in the wrong direction. If the date falls before the anchor, move backward from the doomsday, not forward.