You are looking for a locale override for the
start of week feature of your locale.
As you mention, Evolution provides such an override.
The gtk+ library no longer provides a way to override the locale information for the start of the week, http://library.gnome.org/devel/gtk/2.21/GtkCalendar.html#GtkCalendarDisplayOptions
Since GTK+ 2.4, this option is deprecated and ignored by GTK+.
The information on which day the calendar week starts is derived
from the locale.
Adapting your current locale is not a difficult task, and you may have to go through this venue to solve the problem. That is, either change your locale to, let's say, en_US (start of week is Sunday) or modify your existing locale.
To modify your locale
gksudo gedit /usr/share/i18n/locales/en_GB
and change the values for first_weekday and first_workday. Sunday is
1, Monday is
Then, regenerate the locales with
and finally logout and then login again for the changes to take effect.
Note that if there are any updates of the
locales package and you install them, you have to apply the above changes again.