Let’s say we want to know what day July 4^{th}, 2009 falls on without consulting a calendar. It’s pretty simple, actually. The idea is that you pick a single date each month that you can add to or subtract from in order to get to the date you want. It turns out there are some trends in the calendar that will help us out. The first thing to remember is that April 4 (4/4), June 6 (6/6), August 8 (8/8), October 10 (10/10), and December 12 (12/12) all fall on the same day of the week (and are all *even*). The months in between have related numbers as well. May 9 (5/9) and September 5 (9/5, *think 9-to-5*) are always on that day as well. Also July 7 (7/11, *think seven-eleven*) and November 7 (11/7) are the same. That gives us April through December already without much effort. The last day of February (the 28^{th }or the 29^{th}) is always this day. Since 28 is divisible by 7, that means that on most years (*non-leap years*) both the last day of January and March 7 fall on this day. On leap years the last day of January will be one day off, but all of the rest will be the same. O.K. So now we have a date that is tagged in each month: 1/31, 2/28 or 2/29, 3/7, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, and 12/12. In 2009 all of these days will fall on a Saturday.

Now all we have to do is add or subtract to get any day of the year. Let’s return to our July 4^{th} example. We know that 7/11 is a Saturday so 11-4=7. That’s exactly one week so the 4^{th} is on a Saturday as well. How about September 18^{th}? Well, 9/5 is Saturday so 18-5=13. That’s one week and six days so the 18^{th} is on Friday. April 2^{nd}? 4/4 is a Saturday so 4-2=2. Two days before Saturday is Thursday. Not too confusing, right? The day that all of these dates fall on just makes one step up each year unless it’s a leap year. Last year (2008) it was Friday. Next year (2010) it will be Sunday. So conceivably, one could project these calculations any number of years ahead or back as long as you know which years are leap years. Let’s try it. I don’t know if this will work out so it could be interesting. What day was April 4^{th} of 1973? 2009-1973=36. 36 divided by seven is five with one day left over. Every 4 years you need to skip a day…except on years divisible by 500. Aw, fuck it! It’s faster to look it up unless you are an idiot savant that spends your days rocking back and forth in front of your piano or you’re making the rounds on Oprah and Montel.

So next time you hear some jaggoff whipping days of dates off the top of his head don’t be so impressed. “Oh, January 10^{th} is on Saturday, or May 11^{th} falls on Monday.” Yeah, yeah, yeah. We all know, Smartguy. Ask him what day July 9^{th},1501 fell on. If he throws out a day then kick him square in the sack. The Gregorian calendar (our current calendar) wasn’t created until 1583.

Have a great new year, Everybody!

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