Since all the answers here do not explain why you are facing that problem in the first place, I shall attempt to do so.
when I use != it gives me an error saying "cannot be applied to bool and string"
This is because, in your code:
...vDay != "Monday" || "Tuesday" ...
vDay
is compared to string "Monday" which evaluates successfully and then the result (which is boolean) is compared to string "Tuesday". This gives you the problem of "...cannot be applied to bool and string.."
without the != and just the = I get "string and string
vDay
is being assigned the value of "Monday" (which could result in string "Monday") however the string "Monday" is being compared with string "Tuesday". This gives you the problem of "...cannot be applied to string and string..".
The correct way would be to specify operators separately:
...vDay != "Monday" && vDay != "Tuesday"...
Or using the other ways as best described by other answers.