I calendar window for month we have week started from Sunday. It is may be normal for English people, but for some regions (East of Europe) it is natural to have week that started from Monday. To make it adjustable please add an option to show week start from specified by user day (as it done in Mozilla calendar - Sunbird).
Orage relies in Gtk to show correct week starting day. Take a look on http://bugzilla.xfce.org/show_bug.cgi?id=1328 Most European weeks start on Monday and it works after you have configured your os properly. Coding this would be major effect and will not be done as it is suppose to work already. *** This bug has been marked as a duplicate of bug 1328 ***