The problem is that for some reason that I still need to investigate, the default .po file under env/lib/python3.9/site-packages/django/conf/locale/it/LC_MESSAGES was "corrupted" with all the entries for the day names like this:

#~ msgid "Friday"
#~ msgstr "Venerdì"

Fixing the .po by reinstalling django fixed the issue:

pip uninstall django
pip install django