Skipping data to meet certain criteria...

[Giovanni]

Column A ...Column B ..Column C .......................................Column D
A1 = Mon......B1 = 5 ......C1 = "=A1" .......................................D1= "=B1"
A2 = Tues.....B2 = 6 ......C2 = "=A2"........................................D2= "=B2"
A3 = Wed.....B3 = 7 ......C3 = "=A3" ........................................D2= "=B3"
A4 = Thur.....B4 = 8 ......C4 = "=A4" ........................................D4= "=B4"
A5 = Mon.....B5 = 9 ......C5 = PULL FROM ABOVE (b/c is no fri).....D5= "=D4"
A6 = Tues.....B6 = 10 ....C6 = NEED THIS TO BE MON(A5-B5).......D6= "=B5"
A7 = Wed......B7 = 11.....C7 = NEED THIS TO BE TUES (A6-B6)....D7= "=B6"
A8 = Thurs....B8 = 12.....C8 = NEED THIS TO BE WED (A7-B7).....D8= "=B7"
A9 = Fri........B9 = 13.....C9 = NEED THIS TO BE THURS (A8-B8)..D9= "=B8"
A10=Tues...B10 = 14....C10 = NEED THIS TO BE FRI (A9-B9)......D10="=B9"
..............................C11= PULL FROM ABOVE(b/c is no Mon)...D11="=D10"
..............................C12=NEED THIS TO BE TUES(A10-B10)....D12="=B10"


My problem,,, in Column C and Column D, these columns are in 5 day blocks "From monday to Friday", then it repeats itself. C & D are pulling information from A & B. Is there any way to recognize when days are missing from Column A & B?,,, and then make the necessary correction in columns C&D?

thanks for your time!

Gio