Was it ever disclosed?
It has never been explicitly stated. Most writers will tend to avoid specific DOBs to allow their character's age to remain ambiguous or unchanging. Even the DOB of ageless characters, such as Wolverine, are kept deliberately vague. Establishing a concrete date can screw with the continuity. Especially once the comic series (or any other form of media) runs for more than a few years.
it has been said in a french comic it is 3/4/1978
Deadpool's date of birth is: 11/22/1973