Determine date based on request date meanwhile falls on particular days

I wish to use a formula to determine delivery date based on the request date meanwhile I need it to falls on particular days which allows delivery.
I have been digging around the forum to find relevant formula but I failed to get any. Please advise if you have any suggestion. Thanks~*

Example:

Customer: ABC (Deliver on Thursday).
Req Date: 28 Jun 2013
Del Date: ? --> Outcome should be 27 Jun 2013 as I need the del date to be schedule before the req date & falls on Thurs.

Customer: DEF (Deliver on Monday).
Req Date: 28 Jun 2013
Del Date: ? --> Outcome should be 24 Jun 2013 as I need the del date to be schedule before the req date & falls on Mon.I wish to use a formula to determine delivery date based on the request date meanwhile I need it to falls on particular days which allows delivery.
I have been digging around the forum to find relevant formula but I failed to get any. Please advise if you have any suggestion. Thanks~*

Example:

Customer: ABC (Deliver on Thursday).
Req Date: 28 Jun 2013
Del Date: ? --