Short for "preparation day." Missionaries have one day of the week set aside for preparatory activities such as housecleaning, shopping, laundry, letter-writing, and, oh yes, recreation.
Officially, missionaries are supposed to wear their suits and ties on p-day, except when participating in sporting activities. In
reality, there are no ties in sight on p-day. Officially, p-day starts at 6:30 am and ends at 6:30
pm. In reality, the fun often starts at 10:30 the night before, and doesn't end until until
late in the p-day night.
Bill: Hey Elder John, what are you going to do
tomorrow?
Elder John: Well, it's my P-Day so I'm going to go get some groceries,
clean my apartment and go play some football!