When to activate the pass

Hi,

I will be in Japan 7/11 - 9/18 and would like to use a 7 day pass from 9/2 -9/8. I will be ordering the Pass (Exchange Order) in the United States. Because of the various deadlines, I would like to know when I should activate the Pass., and where. On the day I exchange the pass (prior to 9/2) in Sapporo, can I schedule my Day 1 to be 9/2 and start my journey from Iwamizawa?

Thanks.