Simply change one setting in locale config
sudo vim /usr/share/i18n/locales/en_US
Find section
week 7;19971130;7
first_weekday 1
first_workday 2
...
Simply change one setting in locale config
sudo vim /usr/share/i18n/locales/en_US
Find section
week 7;19971130;7
first_weekday 1
first_workday 2
Set first_weekday equal 2.
sudo locale-gen
Restart (full or XFCE session).