The “Day of the Week” Trick

By Matthew Canning, Become Better at Everything Founder

Updated in April 2017 to make the dates more relevant.

She burst violently into the room. As the door creaked to a close behind her, her eyes adjusted to the darkness. She noticed Luciano, his Walther PPK steadily aimed past her at a point in the distance. She followed its path with her eyes to find two older men—identical in both appearance and attire—pointing weapons back. Realizing that she stood squarely in the crossfire, she backed against the wall and began to breathe heavily. “Regina, my dear,” Luciano said, his eyes remaining fixed on the two men across from him, “this could get interesting.”

He continued speaking as he cautiously closed the distance. “Gentlemen. How do I know which of you is really my father and which is a cyborg?” One of the two identical men—the one farther from Regina—quickly replied, “Simple. Ask me a question only your real father would know.”

Luciano considered the suggestion as he continued his approach. The silence was punctuated only by the echoes of his footsteps. “Fine. What date is my birthday?”

“December eighth.” The answer was almost immediate.

Luciano stopped walking and subtly smiled. He was now almost gun-to-gun with the two men.  “Correct. And what did we do to celebrate it on an icy Saturday this past year?”

“We went with your mother to visit Carol.”

With that, Luciano gunned the imposter down. Sparks erupted from the wounds, exposing stainless steel and a network of tightly-coiled wires. Luciano’s real father lowered his weapon, his lip quivering as emotion overtook him. Luciano embraced the old man. Still reeling from the scene she had just witnessed, Regina found her voice and addressed Luciano.

“How—how did you know?”

He turned to her, raised an eyebrow, and said, “December eighth was on a Thursday this year.”

[End Scene] (applause)

Okay, so nothing remotely this interesting is ever going to happen to you. But still, if presented with a date, wouldn’t it be cool to be able to quickly deduce exactly which day of the week it falls upon? Okay, cool is a strong word. How about incredibly convenient? After just a few minutes of work, you will be able to do so by performing the following trick.

Skills Required: There are two versions of this trick. The first you’ll learn is incredibly simple. You will only need to memorize a single month-to-number index and an easy math trick. After mastering this, you can read on to learn a more advanced version, which will require the Mnemonic Characters and Journey Methods described in the book, Never Forget Again, because you’ll need to memorize six lengthy numeric arrays.

The following method has appeared—in one form or another—in an abundance of literature on topics ranging from Memory Development to magic. There are many different versions of this method out there, but the one found below is both simple and scalable. Once mastered, this skill can reduce your reliance on external tools such as smartphones when performing date-to-day conversions: “Where will I be on November 8th? Well, it’ll be a Tuesday, so I’ll be at work.”

It’s easy and impressive. Let’s look at how it’s done.

The Method in Five Steps

Five steps sounds like a lot, but you’ll be able to go through them very quickly.

Step 1: Convert the Month

Take the month, and convert it to a number using the following table:

  • January = 1
  • February = 4
  • March = 4
  • April = 0
  • May = 2
  • June = 5
  • July = 0
  • August = 3
  • September = 6
  • October = 1
  • November = 4
  • December = 6

Obviously, they aren’t in any order, so come up with Mnemonic Clues to help you recall the respective pairings. For example, you could think of October as involving a candle (Halloween, candle = 1), receiving a gift-wrapped, coiled snake as a Christmas gift (December, snake = 6), and Roman Emperor Augustus wielding a trident (August, trident = 3).

Step 2: Add the Date

Take the date (that is to say, 5 for the fifth of the month, 21 for the twenty-first, etc.) and add it to the month number from step one. For example, since September has a value of 6, September nineteenth would mean 6 + 19, or 25.

Step 3: Adjust for the Year

Let’s just look at the most practical years. Add 6 for 2017 and 0 for 2018. Other years are covered below in the advanced version.

Step 4: Account for Leap Years

This is simple and usually involves no action at all. The next leap year will be 2020. Leap years are covered below in the advanced version.

Step 5: Perform a Tiny Little Bit of Math

Strip out as many sevens as possible from your total. For example, 8 becomes 1, 12 becomes 5, 33 becomes 5, etc. Don’t strip 7 to 0, but instead, just keep it as 7. And now you have your answer. If your final answer is 1, the day was a Sunday. If it is 2, the day is a Monday. 3 = Tuesday, and so on, up to 7 = Saturday.

Try It

Let’s do an example together: let’s determine the day of the week upon which March 2, 2014 fell.

March equals 4. Add the 2 from the day of the month, and we have 6 so far. Add 9 for 2014. 6 + 9 = 15. Since it wasn’t a leap year, we just strip off 14 (all the 7′s) and are left with 1. The final answer, 1, means March 2, 2014 will fall on a Sunday.

Try one on your own: July 1, 2019. Once you’re finished, look the answer up. Were you correct?

Enjoy. It’s a cool little trick that can blow a few minds without much work. And who knows…you may just end up saving the day. But probably not. Now that you understand the process, note that there are two approaches to this. The first is to spend a minute at the beginning of each month memorizing the month-year combo for the next month or so. The second is to go crazy and memorize it all.

The Advanced Version

Uh-ho, we have a bad-ass over here. If you really need to deal with less practical dates, here’s what you must do:

When adjusting for the year, you’re really adjusting for both the century and the last two digits of the year.

When Adjusting for the Century: In most cases, you’ll only need to worry about the current century (2000 through 2099). This adds 6 to the running total. Get used to adding 6 right away. If the year is in the 1900′s, take no action (add 0). If you’re dealing with the 1700′s, add 4, the 1800′s add 2, and the 2100′s add 4. For any other centuries, seriously, look it up. What are you, a historian?

When Adjusting for the Last Two Digits of the Year: As discussed, your best bet is to concern yourself only with the most practical cases (the current year and maybe the next). In order to use this trick for any year, you’ll need to memorize seven arrays of numbers.

  • Add 0: 00, 06, 17, 23, 28, 34, 45, 51, 56, 62, 73, 79, 84, 90
  • Add 1: 01, 07, 12, 18, 29, 35, 40, 46, 57, 63, 68, 74, 85, 91, 96
  • Add 2: 02, 13, 19, 24, 30, 41, 47, 52, 58, 69, 75, 80, 86, 97
  • Add 3: 03, 08, 14, 25, 31, 36, 42, 53 59, 64, 70, 81, 87, 92, 98
  • Add 4: 09, 15, 20, 26, 37, 43, 48, 54, 65, 71, 76, 82, 93, 99
  • Add 5: 04, 10, 21, 27, 32, 38, 49, 55, 60, 66, 77, 83, 88, 94
  • Add 6: 05, 11, 16, 22, 33, 39, 44, 50, 61, 67, 72, 78, 89, 95

For those of you who have read my book, Never Forget Again, memorizing these arrays is a great way to practice using your Mnemonic Character Library. In order to remember these, think of seven short adjoined Journeys, and fill each with one of the above arrays. For example, the first Journey should contain the numbers 00, 06, 17, etc., and the second Journey should contain 01, 07, 12, 18, etc. It would be wise to record each two-digit number solely as individual Mnemonic Characters without using Actions or Single-Digit Mnemonic Clues, so that one Character exists within each Journey Step. Doing so will isolate each two-digit integer such that they are quickly and easily associated with their respective Journeys with minimal translation/decoding necessary. In order to recall which Journey is associated with which number (the number added to your running total), consider starting each Journey off with a Step that contains the Single-Digit Mnemonic Clue of the number. For example, if a local market is your “add 0″ Journey, you could fill the first step with a giant goose egg, which erupts, spewing matter (your Transition) to the second Step of the Journey. This second Step, in this case, would contain your Mnemonic Character for 00 (the first two-digit integer in the array). The third Step of the Journey would contain your Mnemonic Character for 06, and so on.

Once you have this memorized, all you have to do is add the number associated with the last two digits of the year to your running total so far. For example, if 12 were your total so far (month code + day of the month + century adjustment), and the year we are speaking of ends in 66 (such as is the case with 1966 or 2066), you’d add 5 to your subtotal of 12, as 66 is an “add 5″ year.

When Adjusting for Leap Years: If the year in question is indeed a leap year, and the day in question occurs before February 29 of that year, subtract 1 from your running total. If it occurs on February 29th or later, leave it alone. Obviously, most days fall after February 29th. To determine if a year is a leap year, use the following rule (which works for any year after the 1560′s): if a year is divisible by 4, but not by 100, it is a leap year. The only exception to this rule is any year that is divisible by 400. If you think about it, 2000 is the only modern year (1900 or later) that is both divisible by 100 and 4.

Got it?

Let’s do an example together: let’s determine the day of the week upon which January 22, 1969 fell.

January equals 1. Add the 22 from the day of the month, and we have 23 so far. Add nothing for the century, as the date in question occurred during the 1900′s. We then add 2, since 69 (from 1969) is an “add 2″ year. 23 + 2 = 25. Since it wasn’t a leap year, we just strip off 21 (all the 7′s) and are left with 4. The final answer, 4, means January 22, 1969 was a Wednesday.

Well done.