Although we highly recommend focusing on your 7-day rolling averages for continuous progress, for long-term tracking and in your timeline, we take the week as a unit of measure. We had to pick a start time, so our week starts on Monday morning at 3 am and ends the following Monday at 2:59 am.
If you visit detailed stats on your profile, you will see some of your stats broken down on a weekly basis on your graphs. It’s a great place to reflect and make changes to your habits as needed.
However, if you prefer to have the week start on a Sunday, in settings on iOS, within Ate, you can go to "Beginning of the week" and change the default from Monday to Sunday.