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.
You do have the opportunity to select when the beginning of the week should begin. Do you prefer Sunday or Monday or if you would much rather hide it, there is that option as well. Regarding when the end of the day ends, it ends at 3 am and ends the following day at 2:59 am to capture any late-night snacking/activities.
To set your beginning of the week preference: Profile > Settings > App Setup > Beginning of the week.