The book Married Man Sex Life Primer spends a lot of time explaining the menstrual cycle, and how the sex dynamics change around it. One of the suggestions of that book is to keep a calendar of her period, with labels about when it is shark week, but also, when she will be very horny.

Does anyone know any good apps for the phone to aid with keeping your wife's period?

I have an iPhone (but other resources are welcome as well that work with other calendaring and such). I want something that I can just log in when I see first blood, and when it ends, and from logging that have it estimate the period dynamically correcting it. My wife's period is a bit irregular (plus or minus 2 days she says), so something that can adjust to that to recalculate predictions. The least questions I need to ask her, the better. It would be preferable if I can change the labels myself of the different stages (Shark Week, Pound Away, etc).

Update: I understand many of you can keep track your wife your head. Remember that there can be a lot of variability in women's period, and my wife has quite some variability. She has been clear she doesn't like talking about her period at all, she doesn't even track it, and if often in denial about the less obvious sign. Her hamster prevents her from admitting that her drive and moods depend on it. Also that she has said how she expects me to know it better than her. Yes, this is her illogical hamster and her issue. But instead of fighting about it, I can just log it in an app, and use this information to decrease fights and increase sex.