I'm quite sure I read that this festival of Assisi is also celebrated in Florence in May. Is this correct?
Apparently it is. Search "calendimaggio firenze" and you get lots of hits in Italian. Here's the Wikipedia page that you could translate with Google Translate: https://it.wikipedia.org/wiki/Maggio_Musicale_Fiorentino
Calendimaggio was a big feast in Renaissance time, but now there are no peculiar celebrations in Florence - apart from the fact that May 1st is Labor day and a big holiday all over Italy. The Maggio Musicale is a classical music festival held in May and June.