Funny that you should ask that question. I just called today and asked that question to America West staff (I am gold on Am West).

Their rep stated that my Friday 2:30PM flight had been automatically upgraded as of 12:01AM Wednesday morning. Hence, two days prior meant that Friday did not count, and the upgrade posted right after the clock struck midnight Tuesday/Wednesday AM.

Presumably the "two days prior" definition is the same across the board.

P.S. I only rarely fly NW, but, their coach section seems pretty decent.