The “Ave Maria” indicates the change of the religious day from day to night. It is rung half an hour after sunset.
When there were large religious communities in Roman churches and chapters of canons, Vespers would be sung an hour before the Ave Maria Bell. However, in the Roman Curia, Cardinals and other officials would still receive people in audience for the hour after the Ave Maria Bell rang. An hour after the Ave Maria, a single bell would toll, thus ending all business for the day, since it was the first hour of night.
Thus, the Roman Ave Maria Bell.
And, before that, something of the last daylight that gives the City a special color, hard to describe.