How to find the week day of (any) given date? How to find the week day of any given date?
Say we need to know in which week-day was June $25,2019$?
 A: To determine the week-day of a given date, we need to:


*

*find out whether the given year is "common" or "leap".

*know $\mod(a,b)$.

*know $\left \lfloor a \right \rfloor$.

To find out whether the given year is "common" or "leap", we can use the following chart:

$\mod(a,b)$ means the remainder when dividing $a$ by $b$. For example, when we divide $17$ by $3$, we get $5$ and the remainder is $2$. Therefore, $\mod(17,3)=2$.
For convince, $\mod(a,100)=$ the number formed by the last two digits of $a$. For example, $\mod(13527,100)=27$.

$\left \lfloor a \right \rfloor$ means the nearst integer less than or equal to $a$. For examples,
$\left \lfloor 6.97 \right \rfloor=6$,$\left \lfloor -2.8 \right \rfloor=-3,\left \lfloor \frac{20}{4} \right \rfloor=5$.


Suppose that the given date is of the form: MONTH $d, y$

We have to calculate the following:


*

*$A=\mod(y,100)$

*$B=\left \lfloor \frac{A}{4} \right \rfloor$

*$C=\frac{y-A}{100}$

*$D = d$ which is the given date.

*$E =\left \lfloor \frac{C}{4} \right \rfloor$

*$F=0,3,2,5,0,3,5,1,4,6,2,$ or $4$ depending on the given month (Jan, Feb, March, ..., or Dec) respectively.

*$G=\left\{\begin{matrix}
0 & \text{if the month is not Jan nor Feb}\\
1 & \text{for Jan or Feb in a common year}\\ 
2 & \text{for Jan or Feb in a leap year}
\end{matrix}\right.$

*$H=\mod(A+B-2C+D+E+F-G,7)$

*The week-day depends on the $H$ value,
$0$ for Sunday, $1$ for Monday, $2$ for Tuesday, $3$ for Wednesday, $4$ for Thursday, $5$ for Friday, and $6$ for Saturday.


Consider the example, June $25,2019$

Since $2019$ is not divisible by $4$, then $2019$ is a common year.
$A=\mod(2019,100)=19$
$B= \left \lfloor \frac{19}{4} \right \rfloor=4$
$C=\frac{2019-19}{100}=20$
$D= 25$ as given.
$E= \left \lfloor \frac{20}{4} \right \rfloor=5$
$F=3$ for June.
$G=0$ since the given month is neither Jan nor Feb.
$H=\mod(19+4-2\times20+25+5+3-0,7)=\mod(16,7)=2=$ Tuesday.

I noticed that many people ask about this. I posted this way because I think it is the simplest way for any given date , whatever the given century.
There are some simpler ways but for years between 2000 and 2099 only.
So it is a general way.
If you know any simpler way than this, please leave a comment or just post it as an answer, THANKS!.
This may be a useful page for you: https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week
