By default, the start of the week in the OnePlus smartphone's Calendar app is set based on the selected region.

For example, if you're traveling abroad for business, you can set the start or beginning of the week at any time, with the following options available to us

Android 10 | OxygenOS 10.0
Step 1:  Open the  Calendar app
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  General
Step 5:  Tap on  Start of the week
Step 6:  Choose  Saturday ,  Sunday  or  Monday
  1. Open the Calendar app
  2. Open the menu
  3. Open the Settings
  4. Tap on General
  5. Tap on Start of the week
  6. Choose Saturday, Sunday or Monday

OnePlus Instructions