The word sagra denotes a local fair, usually a celebration of the bounties of the earth, meaning food as in a preparation (sagra di torta di erbe) or a raw ingredient (sagra di pesce [fish]). There is probably a sagra somewhere for nearly every food found in Italy.
Attending a sagra is a way to get a taste of Italian country life and food culture and get away from tourist crowds. You order food to be cooked by locals with a passion for the local cuisine, then sit at communal tables with other locals to eat.
Eating at a sagra is usually inexpensive as well.
Larger sagre (the plural of sagra) may feature musical bands or stands selling local foods and other items. Sometimes there's a competition of some sort, like a bike race, but these are usually found at a festa, or festival.
How to Find a Sagra
You'll probably need a car to attend a sagra, as most of them are held in small towns, usually in the main piazza or a specially designated area where there are tables - just follow the crowd. When you walk through small towns or drive through the Italian countryside, you'll see colorful posters posted at intersections indicating the sagra di ____, with the dates and times large enough to be read from a passing car. Some sagra are also held in cities and you can find them the same way, by looking for posters.
The posters are in Italian, of course, but they're pretty easy to figure out. See Reading a Sagra Poster to see the kinds of information you can glean from one.
Most are not publicized on the internet although some bigger ones are beginning to have web sites or Facebook pages.
When to Go to a Sagra
Sagre are usually held at dinner on Friday (venerdi) and Saturday (sabato) and lunch on Sunday (domenica), but this may vary so check the poster. Most sagre are held for only one or two weekends each year.
Many sagre are held starting in late spring and during summer but fall is also a good time to find them. In fall, chestnuts (castagne), mushrooms (funghi), and wine or grapes (uva) are common and in some places in northern and central Italy you will find truffles (tartufi), the ultimate foodie treat, although they are usually referred to as a truffle fair or festival.