What happens if it rains on the concert night and you have lawn seats?

Ring the theatre and ask what arrangements they have in place for this. They might have seats available undercover, or set up tarpaulins, or might provide seat covers and large umbrellas. If not, you could read the fine print on your tickets or on the theatre's website to see if you can request a rebooking or a refund. If you really want to see the concert, take large umbrellas and something to cover the seats, like a car rug. Bear in mind that big umbrellas can be a problem in a crowd and you'll need to be very considerate.