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 ...

"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."

"By default, windows will show Sunday to Saturday... if you want to change it to Monday to Sunday follow these steps.

Step-1 Open control panel (Windows button+X, Select control panel (P))

Step-2 In Clock, Language and Region select Change date, time, or number format

Step-3 In formats tab go to the first day of week select Monday (or whatever the
day you would like to start your week)

Step-4 Press Apply and OK.

Now check your system calendar. it's done :)"

"Reg script

Adding to user8228's answer:

Create a .reg file with the following content:

Windows Registry Editor Version 5.00

[hkey_current_user\control panel\international]

""ifirstdayofweek""=""0""

; the change is immediate

Either click on it to be imported using graphical confirmation dialog, or use reg import *.reg command to do the same from the command-line.

Note

case doesn't matter except in the first line
REG_SZ (string) type is indicated by quotes (""0"")
double quotes are mandatory as single quotes won't work
reg command is the reg.exe executable in %windir%\system32
; is for adding comments (commenting the line) within the reg file"