Monday, March 04, 2019

The Doomsday Rule

This rule says that the following dates - 
  • January 3rd (or 4th in a leap year)
  • Last day in February
  • April 4th (4/4)
  • June 6th (6/6)
  • August 8th (8/8)
  • October 10th (10/10)
  • December 12th (12/12)

  • will always fall on the same day of the week.  This is a very simplified version - the full rule also covers the odd-numbered months, but even this stripped-down version will enable you to calculate the day of the week any given date will fall on, provided you know the key day for the year.  The rule also tells you how to find this out, but it's a bit long-winded to explain.  Suffice to say that for this year - 2019 - the key day is Thursday.  So, try it out.  Today is the 4th of March. We know that the last day in February was a Thursday, so four days on means today must be Monday.  Well, what do you know?

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