I do not know if the formatting is corrupted due to pasting it to your question or not, but the spacing/level in front of the lines is important and broken in your question.
Also, at the top your locale is required (and missing in your example).
it:
date:
formats:
default: "%d-%m-%Y"
short: "%d %b"
long: "%d %B %Y"
# Note that the spaces are reduced again
day_names: [Domenica, Lunedì, Martedì, Mercoledì, Giovedì, Venerdì, Sabato]
abbr_day_names: [Dom, Lun, Mar, Mer, Gio, Ven, Sab]
For more information about I18n and Date/Time formatting, see this article. The en.yml shows all available fields you could translate into Italian.
On script/console
you can try the Italian locale:
>> I18n.t 'date.abbr_day_names'
=> ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]
>> I18n.locale = "it"
>> I18n.t 'date.abbr_day_names'
=> ["Dom", "Lun", "Mar", "Mer", "Gio", "Ven", "Sab"]