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What The Day Of The Week Calculator Works Out
- Day of the week: the weekday of any date in the modern (Gregorian) calendar, the classic "what day was I born" question. Any date advances one weekday per ordinary year and two after a 29 February passes, which is why birthdays drift forwards through the week and repeat on the same day every 5, 6 or 11 years.
- Day of the year: how many days into the year the date falls, out of 365 or 366. Leap years are every year divisible by 4, except century years not divisible by 400, so 2000 was a leap year and 1900 was not.
- Fun fact: the fortune-telling rhyme (Monday's child is fair of face and the rest), first recorded in the 1830s and responsible for generations of Wednesday-born children being told they are full of woe. Quoted strictly for entertainment.
- Read the date as (advanced): for dates before 14 September 1752, when Britain skipped 11 days switching calendars, historical documents give Old Style (Julian) dates. Choose Julian and the weekday and leap-year rule follow the old reckoning, so a date from a Tudor letter comes out right. Russia kept the Julian calendar until 1918, which is why the October Revolution happened in November.
- ISO week number (advanced): the week of the year under the ISO 8601 standard used by UK payroll and project planners, where weeks start on Monday and week 1 contains the first Thursday of the year. The first or last days of a year can belong to the neighbouring ISO year.
- Days from today (advanced): how many days ago the date was, or how far ahead it is, with the same figure in years and weeks.
- Doomsday for that year (advanced): John Conway's mental-calculator anchor. In any year 4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 7/11, 11/7 and the last day of February all share one weekday, the year's "doomsday". Learn it and any date is a small offset away, a genuine party trick that takes an evening to master.
- Next year on the same weekday (advanced): the next year in which the same calendar date lands on the same weekday, useful for planning a birthday party on the day itself. The full calendar repeats every 400 years, in which, pleasingly, the 13th falls on a Friday slightly more often than any other day.
- Year-by-year table (advanced): the weekday of the date in each of the next 10 (or up to 50) years, with the matching years marked. A 29 February birthday shows the years with no leap day.