[whpalmer4]

Try this:

Duplicate the project file
Open the duplicate
Bring up the project information inspector (command-4)
Change the start date to be about today
Look at the resulting finish date in the line along the bottom of the project window and compare it to your desired finish date. Iteratively adjust the start date until you hit the target.

Now go look the resulting project over carefully, because there are things that are likely to have shifted around due to holidays, starting on different days of the week, and all the other stuff that makes one week not just like every other week.

Working backward from a fixed end is a good thought, but it works only if the original project was constructed that way, too.