1) backup your current locale

cp /usr/share/i18n/locales/en_US /usr/share/i18n/locales/en_US.bak

2) change first_weekday value from 1 to 2

gedit /usr/share/i18n/locales/en_US

3) generate new locale

localedef -i en_US -f UTF-8 en_US

4) re-login and voil'a :)