Some users prefer to display the start of the week on either a Sunday or a Monday. You can set this preference to any day of the week at a global level. Users can optionally override this in their personal preferences.
Go to Admin > General Settings and set the "First Day of the Week" option.
If you wish to override the global preferences then click on your name in the top panel and select "My Profile". Scroll down until you see the "First Day of the Week" option and set your override preference.