Four Ways to Find Out What Day a Date Falls On

Being able to instantly select the day of the week for a given date from your head may seem like a magic trick, but it’s actually not that hard – if you’re willing to do some work and do some math.

If you want to impress people with your human-computer calendaring abilities, below are four ways to find out the day of the week any date falls on almost instantly. I’ve ordered them from the least complex no-math method to the advanced and ominous-sounding “doomsday algorithm”.

A math-free way to instantly find any day of the week

This is an absurd cheat, but if you want to know what day it is on a specific date, ask your phone.

Obviously, no one will be impressed if you take out your iPhone and ask, “Hey Siri, what day of the week was January 25, 1994?” But if you use a little sleight of hand and tech tricks, 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 iPhone, make Siri respond silently by going to Settings > Accessibility > Siri. Depending on which system update you have, choose Automatic (and put your iPhone on silent) or Prefer Quiet Reply to have Siri always respond silently.
• On Android, ask Google Assistant to “Open Assistant” or go to Settings > Apps > Assistant > View All Assistant Settings. Then select “Assistant voice and sounds” and in the “Speech output” section, click “Phone”. Now select “None (except speakerphone)”.
• Bet someone for a beer that you can instantly name the day of the week they were born.
• Keep your phone out of sight, but hold down the side button (or whatever button you use for Google Assistant on Android) to have your phone listen while your victim tells you the date. If you need, you can repeat the question to them (of course, for your phone).
• Take a sneak peek at your phone and Siri/Google will silently provide the answer you’re looking for.
• Collect your free drink.

An almost math-free way to instantly find any day of the week (without using your phone)

This method of determining the day of the week only works a few months ahead or backward, so its value is limited, but it’s simple and requires almost no math. It only requires a little memorization. Here’s how it works:

• Look at the calendar and note what day the 7th falls on. In August 2023, it is a Monday. So you know it’s Monday 14th, 21st and 28th August.
• Remember that August is the month of Monday. In 2023, September is the “Thursday month”, October is the “Saturday month”, and November is the “Tuesday month”. How many of them you can remember depends on your dedication.
• When someone asks, “What day of the week will November 12th be?” you only need to remember that November is the month of Tuesday, so the 14th is Tuesday. Subtract two and you know November 12th is Sunday.

Mathematical method to name any day of the week

Okay, below is a real way to find out the day of the week for any date, which I stole 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 = 1
• February = 4
• March = 4
• April = 0
• May = 2
• June = 5
• july = 0
• August = 3
• September = 6
• October = 1
• November = 4
• December = 6
• In leap years, January = 0 and February = 3.

Once you’ve committed this 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 to this the day of the month.
• Add the month code above.
• Divide by 7.
• The first digit of the remainder is the day of the week, where Sunday is 1, Monday is 2, and so on. If there is no remainder, it is Saturday.

There are a few special rules for this method:

• The week before 1900, add 2 to the sum before dividing; before 1800 add 4.
• From 2000 to 2099, subtract 1 from the sum before dividing.

So if someone asks what day was May 7, 1964, you have to add 25% of 64 to 64 to get 16. Add 7 to that to get 23. Add “month code” 2 to get 25. Divide by 7 to get 3.5. The remaining 5 means it was Thursday.

This method works after fashion but lacks elegance. It is a blunt instrument with arbitrary memorization. Also, this only works for dates before 1753. So we need to dig deeper.

Use the “Doomsday Algorithm” to determine any day of the week

Despite its name, the Doomsday Algorithm isn’t scary unless you’re afraid of math. The doomsday algorithm, which is an improvement on the formula invented by Lewis “Alice in Wonderland” Carroll, was developed in 1970 by mathematician John Horton Conway. This is partly based on the fact that certain dates in the Gregorian calendar always fall on the same day of the week as other specific dates in the same calendar year. These are Judgment Days.

If you remember that 4/4, 6/6, 8/8, 10/10 and 12/12 (even months) fall on the same day of the week in any given year, you’ve covered most of the calendar. But you can also add other easy-to-remember days. 11.07, 7.11, 5.09 and 09.05 also fall on the same day of the week. (A simple mnemonic: “I work 9 to 5 at 7/11.”) You can also add July 4th, Boxing Day (26/12), Halloween (31/10), and Pi Day (3/11). 14). In 2023, all these dates are Tuesdays.

Leap years complicate things a bit. January 3rd is Doomsday in a normal year. In a leap year it is January 4th. For February, Doomsday is the last day of the month: 2/28 if it is a regular year; 2/29 if the year is a leap year.

From there you will have enough information to quickly calculate the day of the week in any month by adding or subtracting a few numbers from the dates you know. In 2023, all Judgment Days were Tuesdays. So if someone asks, “What day was June 3, 2023?” You may remember that 6/6 is Tuesday, and if you subtract 3 days, you get Saturday. It’s a little more difficult in odd months, but still doable.

“But this only works if you know the day of the week of Doomsday in a given year,” you might say. You are right. But there are several ways to find out. The explanation of how they work gets a bit complicated and lengthy, so let me direct you to this video from Be Smart for a very good explanation of one of the ways it works. Or read the Wikipedia page for many more options.

Once you figure out the method that works for you, you can do it in your head. With a little practice, you can name any day of the week for any date in history in a matter of seconds , which seems to be impressive enough to get into Asia’s Got Talent .