We have tickets for James Taylor, actually on the 3rd. How early should we plan to arrive to get decent lawn seats?
If if starts at 7, I would be there no later than 5:30. Bring a picnic and a bottle (or two) of wine and enjoy the atmosphere. There really aren't any bad lawn seats as there are large screens where you can watch the performers, of course that really doesn't help until the sun goes down. You will be able to hear and enjoy the music from any spot on the lawn.
You don't stay where you are coming from but beware of traffic heading there especially getting off the MassPike. While it has certainly gotten better, there can be long delays that day going to the concert.
I worked for the Pike the year they instituted the FastLane (now called Easy Pass) and nobody had it. We had a miles long back-up for people going to Taylor on the 4th. Some people missed the show due to the long backup. This situation has improved drastically with the rebuild of the exit ramp onto Route 7 but just beware of it and plan accordingly. Also, if you see heavy traffic at Exit 2, go to Exit 1 and follow directions to Tanglewood from there.