Quickly Calculate the Weekday for Anyone's Birthday
You don't have to be very smart to calculate the day of the week for any date -- 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: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: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 EO came 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.