In Mama Day, I don't think literal circles are so important to the story. The few times circles are mentioned, they don't seem too critical. However, if you want to talk about how circles in a figurative or symbolic sense are important, that's something I can tell you about.
If someone is moving in a circle, it usually means that they’ve moved all the way around something just to go back to it. In other words, if you're moving in a circle, you're always going back to where you started.
I think you could say that Cocoa is moving in a circle. Each August, Cocoa returns to Willow Springs. She goes back to her home. You could argue Cocoa's circular motion links to George's death. Think about the set of events that lead up to her husband’s downfall. It seems to all start with Cocoa circling back—returning to—Willow Springs.
When Cocoa goes back to Willow Springs, she goes to a concert with Junior Lee. As Ruby and Junior Lee are together, Ruby becomes jealous and turns to magic to get some vengeance.
Perhaps it wasn't Ruby's magic that led to George's downfall. Perhaps it was something more normal, like George's inability to get proper healthcare for his heart condition.
That, too, could be linked to circles and Cocoa's circular tendency. If Cocoa wouldn't have brought George back home with her, maybe George wouldn't have died.