My lettuce was pretty much finished by June this year, as it has been especially hot and dry in Central Texas. I believe this is the worst drought on record since the devastating drought of the early 1950's.
I have heard people claim to grow lettuce in the summer in south central Texas and have wondered how they get the seeds to germinate when daytime temps are in the 100's, and nighttime minimums are in the high 70's. Since no one has ever given me a straight answer, I suspect they were purchasing seedlings at a plant nursery.
Meanwhile, I found some interesting info on priming lettuce seeds to promote germination even in warm weather: Why prime seed? by H.J. Hill, Seed Physiologist, Seed Dynamics, Inc.
Among other things, the article explains why lettuce seed requires both light and cool temperatures to germinate. It goes on to say that one can achieve good germination even at high temperatures is the seeds are primed.
The article was written for commercial growers and does not offer techniques home gardeners can use to prime seeds. What I do is fold a paper towel in half and spray it with water until it is damp but not soaking wet. Then I put the lettuce seeds on one half of the exposed surface of the paper towel, and fold the other half over. The paper towel is thin enough to let light through, which is important, as lettuce seeds need light to germinate. To keep the towel moist, I put it into a plastic bag, but I make sure the bag is open. The seeds will die if they do not get oxygen, same as an animal would die if you put a plastic bag over its head.