Being able to instantly pluck the day of the week for a given date out of your mind can seem like a magic trick, but it’s actually not that difficult—if you’re willing to put in a little work and do a little math.
If you want to impress people with your human-computer-like calendar abilities, below are four ways to nearly instantly know the weekday any date falls on. I’ve ordered them from the least complicated, no-math method to the advanced and ominous-sounding “Doomsday Algorithm.”
The math-free way to instantly find any day of the week
This one is cheating to the point of absurdity, but if you want to know what day it is on a specific date, ask your phone.
It’s obviously not going to impress anyone if you whip out your iPhone and say, “Hey Siri, what day of the week was it on January 25, 1994?” But if you use a little sleight of hand and tech trickery, you can use Siri or Google Assistant to make it look like you’re a human calendar, which should be convincing enough to at least win a quick bar bet.
Here’s how to do it:
On your iPhone, get Siri to respond silently by going to Settings > Accessibility > Siri. Depending on what system update you have, opt for “Automatic” (and put your iPhone on silent mode) or “Prefer Silent Response” to have Siri always respond silently. On Android, ask Google Assistant to “Open Assistant,” or head to Settings > Apps > Assistant > See all Assistant Settings. Then, choose “Assistant voice & sounds,” and, under, “Speech output” tap “Phone.” Now, choose “None (unless hands-free).”Bet someone a beer that you can tell them the day of the week they were born instantly.Keep your phone out of sight, but hold down the side button (or whatever button you use for Google Assistant on your Android) so your phone is listening while your victim gives you the date. If you need to, you can repeat the question back to them (for your phone’s benefit, of course).Sneak a look down at your phone and Siri/Google will have silently provided the answer you seek.Collect your free drink.
A nearly math-free way to instantly find any day of the week (without using your phone)
This method of finding a date’s day of the week only works for a few months ahead or behind you, so it’s of limited value, but it’s easy and nearly math-free. It only requires a little memorization. Here’s how it works:
A math-centric method to name any day of the week
Okay, below is a real way to figure out the day of the week from any date that I swiped from the Old Farmer’s Almanac. It takes some math and memorization, but it’s the kind of math you can do in your head.
Remember this code for the months of the year:
January = 1February = 4March = 4April = 0May = 2June = 5July = 0August = 3September = 6October = 1November = 4December = 6During leap years, January= 0 and February = 3.Once you’ve committed that to memory, do the following:
Take the last two digits of the year and add one-quarter of those two digits. Ignore any remainder.Add the day of the month to that.Add the month code listed above. Divide by 7.The first digit of the remainder is the day of the week, with Sunday being 1, Monday being 2, etc. If there’s no remainder, it’s Saturday.There are a few special rules for this method:
For a week prior to 1900, add 2 to the sum before dividing; prior to 1800, add 4.From 2000 to 2099, subtract 1 from the sum before dividing.So if someone asks what day was it on May 7, 1964, you would add 25% of 64 to 64 to get 16. Add 7 to that to get 23. Add the “month code” of 2 to get 25. Divide by 7. To get 3.5. The remainder of 5 means it was a Thursday.
This method works after a fashion, but it lacks elegance. It’s a blunt instrument with arbitrary memorization. Plus, it only works for dates going back until 1753. So we need to go deeper.
Use the “Doomsday Algorithm ” to figure out any day of the week
Despite its name, the Doomsday Algorithm isn’t scary, unless you’re terrified of math. An improvement of a formula invented by Lewis “Alice in Wonderland” Carroll, the Doomsday Algorithm was devised in 1970 by mathematician John Horton Conway. It partially relies on the fact that specific dates in the Gregorian calendar always fall on the same weekday as other specific dates in the same calendar year. These are the Doomsdays.
If you remember that 4/4, 6/6, 8/8, 10/10, and 12/12 (even-numbered months) will all fall on the same weekday in any given year, you’ve covered a lot of the calendar. But you can also add other easy-to-remember days. 7/11, 11/7 and 9/5 and 5/9 also fall on that same weekday. (A simple mnemonic device is to say, “I work 9 to 5 at 7/11.”) You can also add July 4, Boxing Day (12/26), Halloween (10/31), and Pi Day (3/14). In 2023, all these dates are Tuesdays.
Leap years make things a little more complicated. Jan 3 is a Doomsday in a normal year. In a leap year, it’s January 4. For February, the Doomsday is the last day of the month: 2/28 if it’s a normal year; 2/29 if it’s a leap year.
From there, you have enough information to quickly calculate a day of the week in any month by adding or subtracting a few numbers from the dates you know. In 2023, all the Doomsdays were Tuesdays. So if someone asks “what day was it on June 3, 2023?” You can remember that 6/6 is a Tuesday, and subtracting 3 days gets you Saturday. It’s a little trickier in odd numbered months, but still doable.
“But this only works if you know the day of the week of the Doomsdays in a given year,” you might be saying. You’re right. But there are a number of methods for figuring that out. The explanation of how they work gets a little complicated and lengthy, so let me direct you to this video from Be Smart for a very good explanation of one way of working it out. Or read the Wikipedia page for a lot more options.
Once you figure out the method that works for you, it’s possible to do in your head. If you practice a bit, you can name any weekday on any date in history in seconds, an impressive enough feat, apparently, to get you on Asia’s Got Talent.
