Dates for 2016 are not yet known officially - you should follow their FB page to get updates - https://www.facebook.com/proradda/ I've already asked on their but see no reply yet.
The Greve in Chianti wine festival is usually in September.
What I suggest is to come when you can -- there are other wine related events in May - http://www.chianti.com/what-to-do-in-chianti/may-wine-events-in-tuscany.html - and once June comes along, there are many events hosted in the Casa del Chianti Classico convent in Radda and around the area during the entire summer. Most are on the weekends. So rather than try to plan for one specific festival for which we won't know dates for certain for a while (unless the proloco answers back), then just enjoy what you do find. Lots of wine and tastings in the area!
two of my favorite wine festivals include a very small but quaint festival at Lamole in the beginning of June. There are only the producers from the Lamole area in Chianti and the town is very small - but that is what makes it so special and not very touristy. (It's called I Profumi di Lamole)
Then the end of August in Poppi in Casentino, this fair is fabulous because the whole town partecipates and many historic palazzos and gardens are open to the public where producers come in and set up shop in the wine cellars of the building and you tour from house to house - so not only do you get to taste some great wine BUT you get to visit one of the prettiest little towns in Tuscany ;-) (It's called Gusto dei Guidi)