Use an if statement to populate the X, but the X will not be there tomorrow as the today is not the same. Not sure if the X is suppose to stay or not, but you could try:
=if(A1=today(),"X","")
Change A1 tot he correct cell and input this formula where needed. If you need the X to stay you could maybe try:
=if(A1<=today(),"X","")
Hope that helps.