## Introduction: How to Calculate the Day of the Week

So.... you want to be able to tell weekdays of any given date and year..?

In this Instructable I will tell you how to do this. I am sure there are a lot of different techniques. But this works for me. First, you have to understand the principle of it all.

## Step 1: Knowing the Principles.

Its all about different "codes" of both month and year. And you end up with a number between 1 and 7.

** 1** - Monday

**2** - Tuesday

**3** - Wednesday

**4 **- Thursday

**5** - Friday

**6** - Saturday

**7**- Sunday

The formula (simplified) will be :

** (Number for the date) + (Code for the month) + (Code for the year) **

This number you get from this, you have to correct for 2 things:

**Leap-years:**

Dates before 1.march on leap-years - __Subtract 1__

2000 - 2008 - 2012 - 2018 - 2022 and so on..... Every 4. year.

BUT.... important....Only the Century that is dividable with 400.

Like 2000 is a leap-year, but 1900 was NOT.

1600 - 2000 - 2400 - 2600 is leap-years.

**Present/past/ or future Century:**

1500 - Add **0**

1600 - Add **6**

1700 - Add **4**

1800 - Add **2**

1900 - Add **0**

2000 - Add **6**

2100 - Add **4**

2200 - Add **2**

2300 - Add **0**

2400 - Add **6**

** Remember, the **Gregorian calender** that we use today, started 1582. So before this, its no point in knowing what day it was...as nobody knew at the time... ;)**

I will give you a quick example , and tell you how to

memorize the code later.:

**17 . of December 1903 **. The day of the first flight by the Wright brothers in Orville.

17, you can cut subtract with 7, 14, 21,or 28. You can leave this as is, but its easier to handle a small number as its getting larger from the other codes.

So you pull away **14**, and gets the number **3**.

Keep that in the back of your head, and go to the next you have to figure out. The month.

The month December will get you the number **2**. Add this to **3** from earlier and you get **5**.

The code for the year 1904 will give you the number **6**. Add **6** to **5**, and you will get **11**. From **11** we can shorten by **7**, and then get **4**.

The year 1903 was not a leap-year, and the number to add to 1900 years is **0**.

So the number you is left with is **4**. And that will tell you this was a __Thursday.__

## Step 2: The Codes, and How to Use Your Imagination Fully

The codes for the months will be:

January **4**

February **0**

March **0**

April** 3**

May **5**

June **1**

July **3**

August **6**

September **2**

October **4**

November **0**

December **2**

This, you have to memorize in your own way. Its best to use your abstract mind, and associate the number with something you can relate to. As for February and March. Then its really cold in Norway. Near 0 degrees Celsius. May is the 5. Month of the year.

November have NON , and October has 4 ,that is the half of Octo (8) …and so on..!

This is the easy part. Next will be to associate the years with a code from **0 to 6.**

You will get 6 groups , that contains different years.: (The leap-years highlighted)

**Group 0**

09 - 15 - **20** - 26 - 37 - 43 - **48** - 54 - 65 - 71 - **76** - 82 - 93 - 99

**Group 1**

**04** - 10 - 21 - 27 - **32** - 38 - 49 - 55 - **60** - 66 - 77 - 83 - **88** - 94

**Group 2**

05 - 11 - 16 - 22 - 33 - 39 - **44** - 50 - 61 - 67 - **72** - 78 - 89 - 95

**Group 3**

00 - 06 - 17 - 23 - **28** - 34 - 45 - 51 - **56** - 62 - 73 - 79 - **84** - 90

**Group 4**

01 - 07 - **12** - 18 - 29 - 35 - **40** - 46 - 57 - 63 - **68** - 74 - 85 - 91 – **96**

**Group 5**

02 – 13 – 19 – **24** – 30 – 41 – 47 – **52** – 58 – 69 – 75 – **80** – 86 – 97

**Group 6**

03 – **08** – 14 – 25 – 31 – **36** – 42 – 53 – 59 – **64** – 70 – 81 – 87 – **92** - 98

There are many ways to memorize these numbers. I use a system that gives each number two letters that I then can associate with a person or whatever you feel like. But a person is the best choice I think.

From the numbers from **0 to 9** you will have to “translate” the number to a letter :

**0** – o (easy…has to be the letter o)

**1** – i

**2** – t (t for two)

**3** – e (3 looks like a big E mirrored)

**4** – r ( kind of looks like a capital letter R mirrored)

**5** – s

**6** – c

**7** – l

**8** – a ( looks like the capital A ,kind of)

**9** – g

Then, you will get this combination:

(the year)

00 – oo

01 – oi

02 – ot

03 – oe

04 – or

05 – os

07 – ol

08 – oa

09 – og

10 – io

11 – ii

12 –it

13 – ie

AND ...so on....all the way to 99.

You start on the top, and try to get the best and first name or object you can think of. I will try to give some examples you can use if you will :

**00 – oo** – Ozzy Osbourne

**02 – ot** – Oter (The small animal that lives near lakes)

**04 – or** – Orangutan

**05 – os** – Oscar

**07 – ol** – Olga

**08 – oa** – Boa (snake)

**10 – io** – ion (picture a spinning ionized atom)

**12 – it** – Italian (I think of my good friend Andrea Biffi here in Instructables)

When you have manage to put a name or object to each number, we can start to group them. Bear in mind that it took me a couple of weeks to get a suitable combination for each year. If you get stuck with a number, just go to the next one and let your brain get some time to figure this out.

Some combinations is really difficult to use.

Like in Norwegian, we don’t use the letter c in our language.

So for the number 68 - ca , you can also use ka instead . For example you can use CAT, and we would say KATT.

Try to use your imagination. Put the person in some fancy clothes, or even naked. Use colours, and even smell. Everything helps you to remember the number.

Some other tip is to do something special with the LEAP-YEARS.

For example the year **1904**.

This will be 04 – or – orangutan . Put a blinking light on top of the monkeys head , or just imagine an arrow through his head. (In Norwegian the leap-years is called “skuddår” and “skudd” is the same as shot in the English language.)

Another way to use the letters, is to use another letter in front or in between of the letters you have. But i CANT be one of the **o i t e r s c l a g** letters you are using to associate letters and words here.

In example: For the number **11** that is the code** ii**, I get the word P**i**pp**i** (Like in Pippi longstockings, in the Swedish tv series)

## Step 3: Grouping

__Next step is to group them all.__

Imagine **6** places you know well. They don’t need to be close to each other, but it could be smart to put them in an order in some way.

Start with **0**, and imagine a big number 0 on the ground. (Ground zero could be a good place for some to use)

The number **1** could be a place with a big flagpole. Or the Eiffel tower maybe.

I found it better to use Roman letters like **II** and **V**.

The number **3** could be the letter **M** for Mc Donals, or the letter **W** maybe.

When you have 6 different places, its time to gather all of your strange persons and animals..!

Start to write all of the names in single groups. When you read trough the list, hopefully you can get an idea of what they are doing there together.

For example, I use the old bowling alley for my number **2**.

I now get all kinds of people in this big hall. I try to imagine all of them bowling. The number **05** for me is **Os**kar. This was the person in charge here, and was always grumpy. So it all came naturally for me.

In the entrance I put up a BIG Roman number II , and everybody has to go in between the letters.

You will make your own associations, and eventually you are ready to do some code-breaking…!

You will now hopefully understand the theory of this Instructable. And you will know the code number for the months and the years.

## Step 4: The Calculations

Now, its time to use all we know...:

I pick a random date:

**25**. may 20**05**. (No special occasion, I think)

From **25**, I can shorten by **21**, and be left with **4**.

May month means you can plus **5**, and be left with **9**.

Shorten this again with **7**, and we are left with **2**.

The year 2005 . 05 – os – oskar , gives the value **2**. So add **2** more. We now have **4**.

For all the 2000 years, we have to add another **6 **like mentioned earlier**.** So now we have **10**.

Shorten this again with **7**, and we now have **3**.

The year 2005 was not a leap-year, and anyway the date was AFTER 1.march, so we don’t have more numbers to to add or subtract.

So we are left with the number** 3**, that tells us that this is a **Wednesday.**

Easy…!

A good place to check this, is here :

__ Another more difficult example:__

I would like to find the day my father is 75 years old.

He is born 28. of February 1941. So he will be 75 in 2016.

As 2016 is a leap-year, I have to have that in mind. The date is also **BEFORE** 1.of march , so this will also affect the calculations.

First we start with 28 . You can either just cancel the number completely, or take away **21** and be left with **7**.

February will give us **0**. So we have now** 0**. (Or 7...it doesn't matter for the end-result.)

The year 20**16** will give us the number **2**, and we now have **2**.

Since its a 2000 number, we have to add **6**. That gives us the number **8**.

And since its leapyear, **AND** before 1.march , we have to subtract 1.

This will leave us with the number **7** ,whitch is a **SUNDAY.**

## Step 5: Credits

For me, this has been an incredible journey to travel.

I started off by memorizing all of the Periodic Elements. In this way, I got a good start , and this was actually very easy for me to learn.

As long as you can use your imagination, and not be afraid to imagine all sorts of beautiful,grotesque , funny,or just normal people,animals,or objects, you will be able to be able to do this in a couple of weeks.

Stop, and put it away for a while, and then repeat every 2 or 3 months.

I got a lot of help from an expert in Norway called Oddbjørn By.

He is an expert and a world champion i Memorizing cards. As well of having loads of other techniques for better memory in the daily life.

Here is his English webpage:

http://www.oby.no/frontpage/?lang=en

** Good luck....!**

## 4 Discussions

Oh my gosh that's so comprehensive!

Thanks for #12, it's also a number i always liked :-)

I love how much this makes my brain hurt!

Agree ! It actually hurts on the left side of the brain after some hours thinking this way...!

Thanks for that.... I will now be spending the rest of the day mopping my brain up off of the floor after it run out my ears. ?