I found out that the problem reside in very poor localization support by webcal.
I had the chance to try it from an US localized Mac and the calendar is working ok.
The calendar always consider Sunday as the first day of the week.
Localization and setting the first day of the week in preferences only make it WRONGLY OFFSET the calendar, and not recalculate it as it should be.
I'm trying to figure out a robust patch to .js files in coreclient but I hope this will be fixed *very* soon.
(The localization of this product it's really KO! On a brand new Lion Server, I found timezone of webcal HARDCODED as America/Los_Angeles)
The file in which you can change the default HARDCODED timezone is /usr/share/caldavd/lib/python/calendarserver/webcal/resources.py
Here is is the piece of code:
tzid = queryValue("tzid")
if not tzid:
tzid = "America/Los_Angeles"
I don't know if this has been already submitted to Apple as a bug but it definitely should.
I found on MacOsXHints a different place to change TimeZone:
For the first problem, the wrong localization files, I found them ( /usr/share/collabd/coreclient/locales/it.lproj/default.strings) but I'm not yet able to make the calendar re-read it.