I haven't personally gone but I've heard it is a very nice medieval/renaissance fair that takes over the small town and Medici villa.
The pdf program that you can download here has both Italian and English and explains the story of the event: http://www.assedioallavilla.it/programma.htm
Dates for this year are September 12-15!
You can reach it by bus from Prato, which you can get to by bus or train from Florence.
Here's the map for buses from Prato to Poggio a Caiano:
http://www.capautolinee.it/Linee_Ed_Orari/P/167 for the maps and this link for the time tables:
http://www.capautolinee.it/Linee_e_O...raurbane/L/173 (click on the second LAM MT in blue that includes Poggio a Caiano to see the map and then click on the PDF download icon to the right - the pdf address keeps changing every year and this link gets broken otherwise).
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