Everything really happens in Piazza Mignanelli, the southern extension of Piazza di Spagna, and the focus is on the enormous column, topped with a statue of Mary, that was erected and blessed by Pope Pius IX in 1857 to commemorate his promulgation of the dogma of the Immaculate Conception three years earlier. The observances and celebrations take place throughout the day, but the festa really gets rolling in late afternoon -- the Pope is scheduled to come by at 4 PM. Traditionally, the religious part of the festa is celebrated in Piazza Mignanelli. The food vendors and street entertainers are in the northern end of Piazza di Spagna and surrounding streets, but there is a lot of blending. If you have lots of energy and a high tolerance for crowds, you can do as many Romans do, and hit the festa and the opening of the Christmas fair in Piazza Navona on the same day.
So how do I know where the Pope is supposed to be?
I look at his offical calendar of religious events at: http://www.vatican.va/news_services/liturgy/documents/ns_lit_doc_01091999_calendar-sep-dec_it.html