Did you understand how to find the remainder? If yes then fine... If no, then don’t worry let me give you another example: Suppose my addition would have been 21. Then to find the remainder when 21 is divided by 7, I will again find the multiples of 7 till I get the number 21 or a number greater than 21.
7 x 1=7
7 x 2=7+7=14
7 x 3=7+7+7=21
7 x 2=7+7=14
7 x 3=7+7+7=21
In the step above I have got the number 21 which is the required number so no need to go to the previous step. We go back only when we get the number (multiple of 7) greater than dividend (in this case 21). Now to find the remainder I will subtract 21 (multiple of 7) from 21 (dividend). So, our remainder will be: 2121=0.
Now you must have got how to find the remainder. If still not, then go through the examples mentioned above again… Or simply use calculator…
Now, how to find remainder using calculator?? It is simple. Let me tell you using my example.
I want to find the remainder when 19 is divided by 7. So first I will divide 19 by 7 using calculator and I will get 2.71428……
Now for calculating remainder I will need digits before decimal point from the answer we obtained i.e. in my case it’s 2.
So, I will do 7 x 2 and get 14. Then 19 – 14 and I get 5. (In short, we calculated 19 – (7 x 2) =5) And this is our remainder! Simple…Isn't it? So, now you can definitely do it.
Finally coming to our main example (my date: 12^{th} April 1990), I got remainder as 5. I hope you have also got the remainder (surely in between 06).
And that is our final result!
SUN

MON

TUE

WED

THU

FRI

SAT

1

2

3

4

5

6

0

Which day of the week comes on the number 5??
THURSDAY… So, 12th April 1990 was THURSDAY…!!
You can cross check it through your computer calendar or cell phone calendar.
Yes, it was THURSDAY… :)
And what about you?? Which day of the week resembles with your remainder number?? Cross check it for your case also through your computer calendar or cell phone calendar. What?? Is it matching?? Oh..!! It must… Congrats…!! You have done it..!! :)
(Note: If it’s not matching then verify your calculations again and make sure that you choose the valid date from the year 1990 only)
So, now you realized that my trick works. Yes it does… Then it’s time to move to the next chapter and reveal my trick step by step.
First of all, we will summarize the steps we need to follow to find the day from date. Suppose our date is 12^{th} April 1990.
Then following are the steps we need to follow:
 Note down the date (12 in this case).
 Note down the key number for month (April in this case).
 Find and note down the key number for first two digits in year (19 in this case).
 Find and note down the key number for last two digits in year (90 in this case).
 Add all the above noted numbers.
 Divide above addition by 7 and find the remainder.
 Check if the year is a leap year or not.
 Select the day respective to the remainder obtained as per the table given below:
SUN

MON

TUE

WED

THU

FRI

SAT

1

2

3

4

5

6

0

Note: We have not yet discussed how to find if the given year is leap or not.
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