On the day of Jefferson's execution, Grant chooses not to be a witness to the deed. He stays at school, and makes his students kneel down to pray while they wait for news. In the meantime he goes for a walk and sits under a pecan tree to wait. Oh so the students have to kneel (how uncomfortable!) while he gets to take a nice stroll? We see how it is…
Out of the blue, something catches his eye:
Several feet away from where I sat under the tree was a hill of bull grass. [. . .] I probably would not have noticed it at all had a butterfly, a yellow butterfly with dark specks like ink dots on its wings, not lit there. What had brought it there? [. . .] I watched it closely, the way it opened its wings and closed them, the way it opened its wings again, fluttered, closed its wings for a second or two, then opened them again and flew away. [. . .]
Yes, I told myself. It is finally over. (31.40-41)
Grant is meditating, refusing to look at his watch or to really think about anything, until he has some sort of sign that Jefferson has gone. The butterfly's arrival is the first thing that distracts him beyond his own sad thoughts.
It's kind of hard to believe that he was able to see the little black spots on the butterfly from several feet away, but since he mentions the detail it makes us wonder why he brings it up? Perhaps it's that the beautiful, yellow butterfly is still beautiful even though it has some specks on it; maybe life can be beautiful even if it has some blemishes.
Then Grant asks a question: what brought the butterfly to him. He doesn't even consider that it is a random occurrence. He needs to find some meaning, and so he decides that the butterfly must have come for a reason. We can see Grant's belief system shifting ever so slightly: instead of being the questioning cynic, he starts to see a need for believing in a reason, like Tante Lou, Miss Emma, and the Reverend all do.
Finally, the butterfly opens and closes its wings a few times. It's almost like it's waving at Grant —maybe even waving goodbye? The butterfly is, at least the way Grant understands it, a sign that Jefferson has passed on.
And why do you think that Gaines chose a butterfly and not, say, a bumblebee for this image? Maybe it has something to do with changing from an ugly old caterpillar to a beautiful butterfly. The novel is about transformation: Jefferson's transformation into a man, and Grant's transformation from a cynic to a hopeful person. The image of the butterfly is a beautiful one that shows that Jefferson has passed on to something lovely. Not a hog in sight.