If you use a Time Component to graph something like tickets Opened Last Week, you will see that it goes from Monday to Sunday.
Currently, that cannot be changed. Please consider voting for this existing enhancement request if you want the ability to define the week: Define your work week.
From past experience, I know that the same company (same install of Xtraction) may want multiple week definitions. In fact, one person may want multiple definitions. If someone is looking at tickets volumes by week, they may want the typical M-Su or Su-Sa, but if they are looking at Change Orders, they may want the week to go Th-We or Fr-Th (I forget which), to coincide with their Change schedule.