When a person collects cash from his dryer immediately after doing laundry. This term has a different connotation from laundry lottery based on the reliability and frequency of the phenomenon. Laundry paydays are what people who never clean out their pockets get, every week, just like a paycheck. Like a laundry lottery, this produces momentary happiness until the person realizes that they had the money all along.
I can't go drink tonight, but tomorrow is laundry payday, so hit me up then.