Maths Tricks Quickly Calculate Weekday


How about you calculate the exact weekday of birth of your friend by just knowing their birth date, and that too in a matter of seconds. Well, if yes, then check this amazing Maths Maic trick to be a master.

The Magic:

Somehow, you learn the birth date of someone you want to impress. Maybe you're carding her at a 7-Eleven, or getting her license and registration for speeding. Either way, you read the date and absent-mindedly add, "Ah, a Saturday." She looks it up on her phone and, astonished, asks how you calculated that so fast. You turn pale. "D-Did I say that out loud? Please, you must tell no one. I can't go back to the laboratory."


The Math:

  • You don't have to be very smart to calculate the day of the week for anydate -- not when eminent mathematician John Conway already devised a clever shortcut for that, which he (somewhat overdramatically) named the doomsday rule.
  • Crack open a 2014 calendar and you'll see that 4/4, 6/6, 8/8, 10/10, and 12/12 all land on the same weekday: Friday.
  • The same goes for other easy-to-remember dates like 5/9 and 9/5, 7/11 and 11/7, the last day of February, Pi Day, July 4, Halloween, and Michael Jackson's birthday (August 29, as you know). Again, all Fridays.

In 2013, those were all Thursdays:

Maths Tricks Quickly Calculate Weekday


In 2012, they were all Wednesdays. Starting to notice a pattern? Since 2012 was a leap year, in 2011 the "doomsday" jumped to a Monday:

Maths Tricks Quickly Calculate Weekday


  • And so on. So, let's say you're trying to find out the day of the week for July 9, 1987. First you have to figure out the doomsday for that year, by using an important world event as a point of reference, for example. As you probably learned in history class, Captain EOcame out on Friday September 12, 1986, so from there it's easy to calculate that 9/5 was a Friday, too.
  • If 1986's doomsday was a Friday, then 1987's was a Saturday, which makes July 4 of that year a Saturday as well.
  • Therefore, a quick finger count tells us July 9, 1987 was ... a Thursday.

The trick is so easy that Conway was able to mentally calculate any date in less than two seconds.




Free preparation material for Maths Olympiad
Quick Maths definitions that everyone should know
All the Maths tricks that you can not miss 
NCERT Solutions for all the subjects