A few months ago, I was talking with a co-worker in the office (back when we were actually working in the office), and it somehow came up in conversation that another person’s date of birth was September 6, 1995.  My immediate reaction, without even thinking, was to muse out loud, “Huh, a Wednesday.”  She looked at me rather strangely and asked, “Are you serious?”  A quick google search confirmed that September 6, 1995 was indeed a Wednesday.  Her next question to me was, “How did you know that?”

I get asked that question every so often, but I never know how to respond.  I answered her the same way I answer every other time.  “Um, it’s just something I can do.  I can tell you the day of the week for any date.  Within reason.”  I never know how to explain it though.  This invariably leads to further questions, quizzing me on the day of the week for other dates as well.  She asked me her birthdate, November 12, 1996, and after a few seconds, I told her it was a Tuesday.  Correct.

Normally, I can’t respond with the day of the week instantly; it usually takes me a few seconds to do some mental calculations.  Of course, I had instantly mentioned that September 6, 1995 was a Wednesday, so she asked me how I knew that one so quickly without even having to take any time at all to think.  So, I had to come clean about that one.  “I remember that day because it was the day Cal Ripken Jr. broke the Major League Baseball record for consecutive games played with 2,131.  September 6, 1995.  I just happen to remember that it was a Wednesday.”  After revealing that I remembered that specific of a detail, I finally had to admit that I was not normal.

This was just one of several examples that have happened either at work or among circles of friends in which I have revealed a supposedly special ability.  Give Ryan your date of birth, or some other date in history from several years ago (nothing ridiculous, like anything before 1901, when the formula gets thrown off by the fact that 1900 was not a leap year like other multiples of 4 standardly are), and he can tell you the day of the week it was.  I’ve done this from time to time among certain groups of people, just because.  I don’t know why, but some people seem to be entertained by it, so hey, give them what they want, I guess.  For example, perhaps someone will talk about Christmas coming up this year in 2020, and I, remembering that Christmas was on a Wednesday in 2019, will casually mention that this next Christmas will be on a Friday.  Or if someone is trying to think of the last time Christmas fell on a Sunday, well, that would have been in 2016, of course.

I really don’t remember how, why, or when this all started, but people have often asked me how I figure out that sort of thing.  I very rarely give a straight answer because it’s hard for me to explain when asked.  But, as a reward for reading this blog, you are in for a treat – because I’m going to explain it here!

First, the following disclaimer – all of this only works between the years 1901 and 2099.  If you’re the ambitious type and want to figure out any dates outside of this range, then there’s an extra wrinkle beyond what I can reasonably explain here, and some of the logic below won’t necessarily apply.

So here we go.  Today is Friday, May 8, 2020.  This means that May 8, 2021 will fall on a Saturday, May 8, 2022 will fall on a Sunday, and May 8, 2023 will fall on a Monday.  In other words, the day of the week moves forward by one during non-leap years.  However, for leap years, the day of the week moves forward by two, so May 8, 2024 will fall on a Wednesday.  Or, if you want to work backwards, you would subtract the one or two days from the day of the week each year.  For instance, May 8, 2019 was on a Wednesday (due to our just having celebrated a leap year), May 8, 2018 was a Tuesday, and so on.

Every four years, the day of the week moves forward by five.  Since today is Friday, May 8, 2020, this means May 8, 2024 will be a Wednesday, and this also means that May 8, 2028 will be a Monday.  Or, working backwards, this means that May 8, 2016 was a Sunday, and that May 8, 2012 was a Tuesday.

Every 28 years, the calendar year falls exactly the same, no matter what.  The calendar years will also be the same at certain intervals in between, often every 5 or 6 years, although that sometimes gets thrown off by leap years.  But, no matter what, the calendar is exactly the same every 28 years.  So, someone might ask, on what day of the week was the Indiana Pacers’ Victor Oladipo born?  So, to answer that question, Victor Oladipo was born on May 4, 1992.  Just add 28 years.  You are now asking yourself, what day of the week was May 4, 2020?  Well, that was just this past Monday.  Therefore, May 4, 1992 was also a Monday.  Happy belated birthday, Victor.

Or, suppose you are asking, what day of the week was November 5, 1955?  If you are a fan of the movie Back to the Future, you should recognize this date and already know the answer to this question.  So, to answer this question, add 28 years, which gives us November 5, 1983.  This may not be recent enough to do you much good, so let’s add 28 years again, and we get November 5, 2011.  So now we try to figure out that date.

At this point, there are multiple ways to figure this out.  For me, I find that it helps greatly to just remember certain dates that are fairly close in the calendar to the date you’re thinking of.  For instance, since I can remember that Cal Ripken Jr. broke a baseball record on Wednesday, September 6, 1995, I can use this to figure out other dates in September.  For this example, I remember that this past Halloween, October 31, 2019, fell on a Thursday because that’s the day I dressed like a three-hole punch version of Jim Halpert for Halloween.  Using this, we know that October 31, 2015 was on a Saturday (by subtracting five days of the week for every four years we are working backwards), and therefore, October 31, 2011 was on a Monday.  Therefore, November 5, 2011 was a Saturday, which means November 5, 1955 was also a Saturday (which, if you’ve seen the movie, you should just know that anyway).

Okay, so maybe you’re not good at remembering certain facts like October 31, 2019 falling on a Thursday to use that to figure out November 5, 2011.  Well, there’s another way that might be easier if you are mathematically inclined.  Another method to use is figuring out what day of the week January 1 fell on in a particular year, and then using that as a reference point to figure out the date in question.

For example, suppose you wanted to know what day of the week April 19, 2005 fell on.  (This, of course, was the date the Basketball episode of The Office aired, as referenced in a previous post on this site.)  One way to do this would be to figure out January 1, 2005, using the following formulas:

New Years Day 19YY = (YY + YY/4 (round up)) mod 7, where 0 = Sunday, 1 = Monday … 6 = Saturday.

New Years Day 20YY = (-1+ YY + YY/4 (round up)) mod 7, where 0 = Sunday, 1 = Monday … 6 = Saturday.

Using the above 20YY formula for 2005, we figure out that January 1, 2005 = (-1 + 5 + 2) mod 7, which equals 6, or Saturday.  Therefore, by adding the appropriate number of days per month, we know that February 1, 2005 was a Tuesday, March 1, 2005 was also a Tuesday, and April 1, 2005 was a Friday.  Then by adding 18 more days, we know that April 19, 2005 was a Tuesday.

Obviously, you can just google any date you want without having to figure it out in your head.  But if you want to be able to mentally figure out the day of the week a certain date fell on to entertain audiences, that’s how you can do it.  So now you know.  If you really want to dive into this, feel free to watch the video below which came out in 2014 and gives an even more thorough explanation than the one I just gave.  And if you really want to dive even further and figure out the day of the week for any date ever, even outside the range of 1901-2099, there are ways to do that too.  I will let you research those yourself.

Perhaps if you have some extra time on your hands during quarantine, you can try this yourself and then later, perhaps we can have a “days of the week” competition.  Game on!

I’m sure this is the kind of classic material you don’t get on Facebook.