Hi,
here is one suggestion.

Enter this formula in cell K4:
=IF(AND(K$3>=$G4,K$3<=$H4),"x","")

Then copy and "paste special" (only formulas) this formula to ALL other "date cells".

Now there is an "x" in all cells within the advertising interval. You can now easily use conditional formatting to make the cells with an "x" have another colour. Of course you can also from the beginning enter this formula as a conditional formatting (using "where the formula is..." instead of "cell content").

Kind regards
Bjorn