If you are trying to find the numeric day of the week for any date, there are Excel functions that will do this for you.
Key the following into a cell excluding the square brakets  [=WEEKDAY(DATE(2018,5,14))] or if you have a properly formatted date in a cell, let's say "A1", then key [=Weekday(A1)].
The result will be a numeral from 1 to 7, where 1=Sunday and 7=Saturday. This is not what most folk desire, so if you would like the week to start on Monday=1 and end on Sunday=7, key the following formula
[=Mod(WEEKDAY(DATE(2018,5,14))+1,7)] or [=Mod(Weekday(A1)+1,7)].
By adding 1 and using the MOD function with a devisor of 7, the MOD function results in the remainder of the first argument divided by the second argument (7).