I don't know the context of the story, but generally, 'today of all days' is a way of saying this is the last day I would want whatever it is to happen. She came early, and (for whatever reason), this is the last of all days he would want her to be early. It is the most inconvenient day on which she could come early. Likewise if you are depending on someone to arrive on time for a really important event, and they are late, you might say, why, today, of all days, did you have to be late. This is the worst possible time to be late. Hope that helps.