Ok, I tried to do this without the huge IF () operator, but it works. Your date is in A1 format, and your shipping date formula is lower in B1.
=IF(WEEKDAY(A1)=1,A1-6,IF(WEEKDAY(A1)=2,A1-7,IF(WEEKDAY(A1)=3,A1-6,IF(WEEKDAY(A1)=4,A1-7,IF(WEEKDAY(A1)=5,A1-8,IF(WEEKDAY(A1)=6,A1-4,IF(WEEKDAY(A1)=7,A1-5)))))))
Quite a lot of what I did, I found out how many days to subtract from each date when you deducted at least 3 business days, and then subtracted more days until you got to Wed. or mon When is your ...
- Maturity date; then (Days to deduct, to get your desired Mon or Wed Shipment date)
- Sunday; then -6
- Monday; then -7
- Tuesday; then -6
- Wednesday; then -7
- Thursday; then -8
- Friday; then -4
- Saturday; then -5
Hope this helps.
-Scheballs

EDIT
Thanks to Barry Houdini for showing me the CHOOSE () function.
=A2-CHOOSE(WEEKDAY(A2),6,7,6,7,8,4,5)
Quite a few times, when you find the offsets needed to subtract the days from your day of the week, you can use the WEEKDAY value as a position in the list of options. So if our date for the day of the week is Sunday, it is "1", and then our choice of the list of functions, the first one is selected. Our repayment date is then subtracted from this value. Very nice barry, thanks.