According to the release notes of Zimbra 6.0.5 it should now be possible to set the standard on week numbering:
"The first day of the week for Calendar can now by set in the COS or accountPreferences tab from the administration console. Bug 14358"
Although I think I checked thoroughly, I couldn't find said setting. Could someone please point me to where I have to look or confirm that it isn't there, yet?