Granny recalls the day George left her standing at the altar, the day he jilted her. Granny does want to tell George that she lived a full life even without him. But Granny also wants to tell George she forgot him. She begins by remembering that, "For sixty years she had prayed against remembering him and against losing her soul in the deep pit of hell, and now the two things were mingled in one, and the thought of him was a smoky cloud from hell that moved and crept in her head when she had just got rid of Doctor Harry and was trying to rest a minute." After this, Porter writes, "Yes, she had changed her mind after sixty years and she would like to see George. I want you to find George. Find him and be sure to tell him I forgot him." While Granny wants George to think she has forgotten him, the irony lies in the fact that Granny has not forgotten George. She has spent sixty years trying to forget that he jilted her and has been unable to. What she really wants is to let him know he didn't get the best of her, that she did lead a full life without him.
The title of the story is the answer to your question: the day she was stood up at her wedding. The message she would like to send George, her runaway groom, is that she went on with her life without him--and it was a good life: "I got my husband just the same and my children and my house just like any other woman."