if your card has one, it would be printed on it. there would be a set of numbers that could look like 08/10 which is month/year and probably something that says "valid through" or something of the...
eg 12/12 its when your card is run out HAHA
It is the date after which the card account will not be authorized to accept any more charges.
Modern credit cards were invented and first used in 1994.
Due date on your credit card billing is the date that you should pay your balances in order not to have extra charges or fees.