There are many events all year round throughout Tuscany, but many are concentrated during the summer.
Many sagre, or festivals dedicated around a particular local product, start in early summer and continue on through the fall as local specialties (truffles, wine, olive oil, marroni chestnuts) come into season. If you are coming to Tuscany during other periods of the year, don't worry as there is always something to see or do (or eat!) then as well.
For specific events, it is best to contact the local tourism office of the locality you will be visiting.