How to set the first day of the week in System tray calendar?

Using regedit, browse to HKCU\Control Panel\International, look for the iFirstDayOfWeek key and set the value to 0 for Monday, 1 for Tuesday, 2 for Wednesday, etc ...

On Windows 10:

• Right-click on the Taskbar Clock area. Choose Adjust date/time

  

• It opens a dialog, with a Formats section, where there is a display-only summary of options, including First day of week. Click the link Change date and time Formats

  

• On the dialog that now appears, you can choose what one you want on the First day of week combobox.

  

I ran into this exact issue when getting issued a UK-regioned desktop.

You need to go to Control Panel -> Regional and Language Settings and pick a region that starts the week on Sunday, like US.