There are several possible symbolic meanings for Light in August by William Faulkner, and none of them are particularly obvious.
The first possibility is that it is a reference to something that happens at the beginning of chapter twenty. Reverend Gail Hightower is sitting in front of the window just before sunset, amazed at how the
fading copper light would seem almost audible, like a dying yellow fall of trumpets dying into an interval of silence and waiting.
In this scene, the minister finally begins to take personal responsibility for so many terrible things in his life, including driving his wife to commit suicide and being such a poor shepherd to his flock. This recognition of the light here (something he has no doubt seen thousands of times before) might serve as a kind of revelatory light for Hightower. In this light, he is finally able to see himself clearly, free of his past entanglements and self-absorptions.
Another possibility posited by literary critics concerns a phrase used primarily in the South. To be "light" is to be pregnant. This comes from the idea of being "lighter" after giving birth and can refer to both humans and livestock. Faulkner denied this interpretation of the title, according to Hugh Ruppersburg in his 1994 book Reading Faulkner: Light in August.
So, the most official interpretation of the title comes from Faulkner himself in this explanation:
. . .in August in Mississippi there’s a few days somewhere about the middle of the month when suddenly there’s a foretaste of fall, it’s cool, there’s a lambence, a soft, a luminous quality to the light, as though it came not from just today but from back in the old classic times. It might have fauns and satyrs and the gods and—from Greece, from Olympus in it somewhere. It lasts just for a day or two, then it’s gone. . .the title reminded me of that time, of a luminosity older than our Christian civilization.
The title, then, is a tribute to one of Faulkner's memorable and moving experiences with the famous light of his beloved South.