Lewis Carroll – computing the day of the week for any given date (1887)

Original Article ↗ Hacker News Discussion ↗

Mental day-of-week algorithms

  • Several commenters discuss mental methods to compute day-of-week in ~10–20 seconds using small lookup tables (“magic month numbers”) and addition mod 7.
  • One widely used variant:
    • Take years since a base year (e.g., 1900 or 2012).
    • Add number of leap years since that base.
    • Add a month-specific “magic number”.
    • Add the day, reduce mod 7, map to weekday.
  • Conway’s Doomsday rule is compared; some find it faster for dates in the current year but harder across years.
  • Others propose tweaks for the year term (e.g., decade-based tricks, “odd+11” rule) to keep mental numbers small.

Month-number rules and confusion

  • Multiple people struggle with Lewis Carroll’s textual rule for month items, especially the “begins or ends with a vowel” part.
  • Some reverse-engineer that each month’s value is derived from the previous month’s item plus its days, mod 7, with shortcut rules for certain months.
  • Several conclude it’s easier to memorize the final month table than to rely on Carroll’s prose.

Leap years and calendar quirks

  • Handling leap years correctly (including century/400-year rules) is discussed; special adjustments are needed for January/February in leap years.
  • There is concern about dates around calendar transitions (e.g., 1752 in the British Empire, differing adoption years globally).
  • One code example for day-of-week acknowledges it is inaccurate around country-specific Julian→Gregorian transition dates.

Historical calendars and notation

  • Commenters recall pre-1752 English “Old Style” new year starting March 25, Roman March-based years, and month-name origins.
  • Tools like cal 1752 and the odd UK tax-year start are cited as lingering artifacts.
  • Broader point: messy notation (in math or chess history) makes otherwise simple ideas hard to use; better notation radically simplifies reasoning.

Usefulness, meta, and side topics

  • Some see this skill as practically useful and mentally fun; others say its main value is as a party trick.
  • There is brief mention of discomfort with celebrating certain historical figures, and a question whether an LLM could devise a novel algorithm for this task.