Calendar - Long date
 

When I select "day" view in calendar it shows, for instance "12 Tuesday". I
want it to appear as 12 May. Looking at previous posts/threads the advice is
to change the long date in Regional and Language Options. I can see the
format of the long date by clicking on the "customise" button. The format
appears as dd MMMM yyy or d MMMM yyyy. When I try to add another format
(presumably dd MMMM) it does not allow me to save. How do I achieve proceed?
I'm using Outlook 2007 and XP SP2