Thanks all for your quick replies! All solutions worked out great.

Now, suppose I expand on this. In Cell A2, it indicates the day which is an
OFF DAY.

Then, depending on the value of A2, I want to mark a "Y" on one of the cells
from B1 to B31.

How would I combine the solution from before with this one?

Can I do something like this?
B1: =IF($A$1=ROW(), "X","") or IF($B$1=ROW(), "Y","")