I think most now talk about starting the night before (it is usually dosed as a single daily dose for most people, as that maximizes the effect on the nightime breathing problems.)
Agree with Ken.
This is a paraphrase of what is current understanding, at least to some: The kidney-metabolic effects of Diamox might take a few days to work, but they are not the main reason Diamox helps, and why taking it a few days in advance is no longer felt to be needed. Taking it beginning day-before (to get it on board) or even the day-of your ascent seems to work well as the central(brain) and CSF(spinal fluid) effects on respiratory sensors and stimulation of breathing (when asleep mainly) works more quickly
Whew!
some general advice again:
http://www.ismmed.org/np_altitude_tutorial.htm#goldenrules
