Let's start with the basics: Light is usually associated with things like goodness and life, while darkness has opposite connotations of evil and the unknown. It's handy to keep this in mind as we explore the recurring images of light and dark that help us keep up with Granny as she comes to terms with dying. Be warned: This is going to get a little grim.
The first major shout out to light in "The Jilting of Granny Weatherall" comes as Granny recalls a fond memory of lighting lamps with her kids. The narrator explains:
Lighting the lamps had been beautiful. The children huddled up to her and breathed like little calves waiting at the bars in the twilight. Their eyes followed the match and watched the flame rise and settle in a blue curve, then they moved away from her. The lamp was lit, they didn't have to be scared and hang on to mother anymore. (27)
Light is pretty awesome, isn't it? It's beautiful, awe-inspiring, and it helps us feel safer and less afraid. If we imagine the light as a reflection of her spirit, Granny's detailed recollection of lighting the lamps alludes to her struggle to hold on to images of life; at this point in the story, she hasn't quite yet reconciled to the idea of death.
A bit later, we get another image of light, but this time it's less rosy because it's immediately followed by the threat of stormy darkness:
Light flashed on [Granny's] closed eyelids, and a deep roaring shook her. Cornelia, is that lightning? I hear thunder. There's going to be a storm. Close all the windows. Call the children in. . . (57)
This image comes right before Granny's acknowledgment that she's probably not going to be sticking around on earth much longer. It could be said that the image of light that transitions to darkness mirrors Granny's own transition from struggling against death to facing it. Bring it on, Grim Reaper.
Finally, as Granny waits for a sign from God, she fixates on the light coming from Cornelia's lamp. We're told:
The blue light from Cornelia's lampshade drew into a tiny point in the center of her brain, it flickered and winked like an eye, quietly it fluttered and dwindled. Granny lay curled down within herself, amazed and watchful, staring at the point of light that was herself; her body was now only a deeper mass of shadow in an endless darkness and this darkness would curl around the light and swallow it up.(58)
This is hardly the 'light flashing before your eyes' we see in the movies just before someone's about to die, but the important thing to note is that Granny is finally willing to descend into the fading light and ensuing darkness. This tells us that she's coming even closer to accepting death. She ultimately seals the deal in the last line of the story when she takes matters into her own hands:
She stretched herself with a deep breath and blew out the light. (61)