In some regions, people use Monday as the first day of the week, and in some other regions, people use Sunday as the first day of the week. In Outlook, you can set any day as the first day of a week. Please see below for details:
Step 1: Click the "File" tab in the ribbon;
Step 2: Click "Options" from the left menu;
Step 3: Click "Calendar" in the "Outlook Options" window;
Step 4: In the "Work time" section, select a day as "First day of week";
Step 5: Click "OK" at the bottom to close the window.