As anyone who has turned on the radio at noon on Thanksgiving well knows by now, 50 years ago, a minor littering case in the Berkshires launched a chain of events that ended up a beloved song and movie: Alice’s Restaurant. Tonight, Arlo Guthrie will perform a concert at the Colonial Theater in Pittsfield that will be filmed by PBS for an upcoming special. Guthrie is also celebrating the anniversary with a nationwide tour. Guthrie discussed the song’s legacy with WAMC’s Alan Chartock.