The Maine lobster festival is in Rockport, as I recall. I think in late august.
Pretty much everywhere is the same as the next; there are only so many ways to steam a lobster. So go for the full tourist experience and find a place right on the water.
After you dismantle a lobster once you will want to switch to lobster rolls afterward. It's good, but a horrible mess.
My GF is from Texas and this was the first real lobster event.