Arriving and Leaving

This has probably been asked before, but I’m most likely going to have to get to the festival and leave the same day. So my questions are which T line is the easiest to take to get there? I normally park at the Riverside green line and take that into the city. What time should I plan on leaving? (I’m coming from southern NH). And how late does the T run on weekends? I want to be able to see FOB but still make it back to my car.