Formula for finding last delivery date and time

Hello,
My company receives shipments every Wednesday at 4pm and Saturday at 4pm.
I have a list of event times in column A and in column B, I would like a formula to list when the the shipment arrives for that row's event.
In my attached spreadsheet, I've included some examples, with the answer I need in column C

So for an event on Wednesday, June 12 @ 8am, items were delivered on the past Saturday, June 9 @ 4pm
But an event later that day, Wednesday, June 12 @ 7pm, the items were delivered only a few hours earlier, Wednesday, June 12 @ 4pm.

Also a brief explination as to why this works would be helpful for future problems of mine.
This type of time math always confuses me!
Thanks in advance for the help! and let me know if I need to explain my problem differently.
Joe