Glad to hear you've finally a copy of TAW, stalks ... it's definitely worth the hunt for it ... soul food ...
you can start your exercises anytime - if you can't wait to start, then don't wait for Sunday!

The day you first start your morning pages'll usually mark the beginning of week 1 for you. Doesn't hafta be, but it's an idea anyway. Say you started your morning pages on a Wednesday; then you can make your week 1 start from that day to next Tuesday, the day you check-in. The day after, you start week 2. More convenient ... what I like to call "synchronity", lol.