It seems that on day George Michael was in Florence and the Travel Festival conference was going on but none of these fit with your description .
What I think is more likely is this one: on the 10th and 11th of September 2011 the crossbow archers from the Calcio Storico Fiorentino - which is the association all of the participants that dress up for the historical processions in Florence belong to - was having a national crossbow competition near Forte Belvedere and Porta San Giorgio. It is indeed likely they would have included a procession through town... since many other traditional events include a "blessing" to make sure of victory, it is also possible that it occurred for this competition.... so I am thinking this is what you saw. It is therefore unlikely to happen again on the same day in the future... the main events around the start of September are tied around Sept. 8, the birth day of the Virgin Mary where there are lots of purple and white robes between the cathedral and baptistery for celebrations since Florence's cathedral is named in her honor.
Wasn't able to find anything else, hope this helps!