At least an hour before. We usually plan it that way. Your server will pace your meal so that you shouldn't be eating when the fireworks start, and then have the next course ready shortly after the fireworks are done.
Sorry, I am not researching for you to find the time since the time could be different each day. I will say that I did good with a reservation 1 hour before the fireworks. We checked in 15 minutes early and were sat within 5 minutes of check in. We finished our entrées just before the fireworks started.