The end of the year!! Ford wants to keep number 1 in their record books on sales, and make room for the new models.
If you don't mind buying "last years model", it is the way to go.
Usually the biggest rebates from Ford and Ford Credit are offered.
I got 15 G's knocked off my 08, and 14 G's off of my 10. (A plan added) Both bought in late November.