Although we highly recommend focusing on your 10-day rolling averages for continuous progress and for seeing long-term patterns, in your timeline we take the week as a unit of measure.
You can choose which day your week starts on. Prefer Sunday or Monday? Pick whichever suits you, or hide it entirely if you would rather not see it. As for the day, it runs from 3:00 am to 2:59 am the following day, so any late-night meals or activities are captured.
To set your start-of-week preference: Profile > Settings > App Setup > Beginning of the week.