It's crunch time for leaf lettuces. If you planted lettuce early in the season, then by now it's bolting, and since it's laticifers are maturing, it gets disgustingly bitter. Laticifers are the cells or ducts that produce latex, milky sap; hey, the genus is named Lactuca after all. This happens because the shorter nights of June induce a change in growth, a shift from juvenile vegetative growth to a mature reproductive growth, and lettuce is only edible in the juvenile form. One way to push the season a bit, is to plant some lettuce later, and then grow the late lettuce as fast as possible because this seems to prolong the juvenile growth phase the most, although some varieties respond better than others. In particular the oak leafed varieties seem best at retaining the juvenile rosette growth form, but you must watch it closely, ready to harvest at the moment you detect any bolting. After this, you just have to wait until August and start planting all over again.