A few years ago, I was lucky to be in Sant'Oreste (where the better half is from) while there was a festival on. Sant'Oreste is great for festivals, they have a few different ones over the year, and the whole town gets together for a few days to eat, drink and party. I really enjoy all of them.
The Festa dei cacciatori (party of the hunters) is, I guess, some kind of sagre, i.e. local food festival (although I can't find it on this list of Sagre in Lazio). It's usually held in May or June (although this year it's been over the first weekend in July - i.e. right now), and consists of a big marquee (where they have seats and tables lined up like a beer-hall) and a stage, covering their main car park just underneath the main gate to the old town. Basically, you take a seat with friends and wait for someone to take your order. As it's the hunter's festival, dishes on offer revolve around wild boar (cinghiale) and hare (lepre) and are reasonably priced (4-5EUR) for a decent sized dish. Local wine is also available (1EUR a beaker) and tends to be very light and crisp (they tend to be between 7-11%), along with grappa and other local moonshines.
It's a great party, and usually lasts the whole weekend. I can heartily recommend it. All the meat is caught locally (pretty much from same mountain that Sant'Oreste is perched on), and it's a great way to get introduced to local culture, and see a part of real Italy, rather than a tourist trap. I'll mention Sant'Oreste a lot on this blog, as it's the town I visit the most and I genuinely think it has a lot to offer. Obviously, the easiest way to get there is via a car (up SS3 - via flaminia from Rome), although it does have it's own train station (with trains from Piazzale Flaminio -next to Piazza del Popolo, four metro stops from stazione termini on metro A), and is a short bus ride from Stimigliano, where there is another train station (with trains from Tiburtina in Rome)
Sant'Oreste has a few other sagre and feste throughout the year, with the main one being the Festa della Madonna, on the last Sunday in May. Check out this Sant'Oreste Pro-loco website (in Italian) for further information.