On Gnome3, with Italian locale settings, the Gnome calendar reports Sunday as the first day of the week. It looks like that applet is built to take its week format from the current locale settings of the system, this means that the current locale reports Sunday as the first day of the week.
This issue probably affects the sys-libs/glibc package.

locale output:
locale
LANG=it_IT.UTF-8
LC_CTYPE="it_IT.UTF-8"
LC_NUMERIC="it_IT.UTF-8"
LC_TIME="it_IT.UTF-8"
LC_COLLATE="it_IT.UTF-8"
LC_MONETARY="it_IT.UTF-8"
LC_MESSAGES="it_IT.UTF-8"
LC_PAPER="it_IT.UTF-8"
LC_NAME="it_IT.UTF-8"
LC_ADDRESS="it_IT.UTF-8"
LC_TELEPHONE="it_IT.UTF-8"
LC_MEASUREMENT="it_IT.UTF-8"
LC_IDENTIFICATION="it_IT.UTF-8"
LC_ALL=

Steps to Reproduce:

1) login in a Gnome3 DE with Italian locale
2) click on the top panel clock

Actual Result:

the leftmost day (beginning of the week)  in the calendar is "dom" (Sunday)

Expected Result:

the leftmost day in the calendar should be "lun" (Monday)

Proposed Fix:
I was able to fix this issue modifying the file:
/usr/share/i18n/locales/it_IT

locating the LC_TIME section, I added, before END LC_TIME:

first_weekday 2
first_workday 2

and ran:
locale-gen

now it works as expected, probably:
/usr/share/i18n/locales/it_IT@euro
is also broken.