But it really isn't something that happens often at all. There are several non binary genders, but rarely do people switch between them. The closest you'd normally have would be someone who is genderqueer or genderfluid expressing and perhaps feeling more masculine one day than another, but this really isn't the same thing as identifying as a man or a woman that day. While it is possible there are people who switch identification in a fluid manner it would be an incredibly small group of people at best.