An old man takes his wife to the doctor for a checkup.
The doctor wanting to have some fun with the old man tells the old man that the old lady's health is deteriorating and the only cure is sex.
The old man asks, shocked, "Sex?"
"Yes", says the doctor.
The old man then asks, "How many times must she have sex?"
The doctor says, "Three times a week."
"What days?" the old man asks.
The doctor ponders and then replies "Well, lets say Tuesday, Thursday and Saturday."
The old man replies a little worriedly, "Well doctor, Tuesday and Thursday I can but, on Saturday I have something to do - I can't bring her to you."