Christmas Special 1974 (a.k.a. A Perfect Christmas) - S8-E7
Corrected entry: In this episode it is revealed that Albert never knew who his father was, as it reads "unknown" for father's name on his birth certificate. But in previous episodes such as "A Winter's Tale", Albert mentions he has a sister in Stoke-on-Trent, and in "Oh What a Beautiful Mourning", Albert came from a family of 14 children.
Correction: None of that means that he has to know who his father is. If his mother was promiscuous, it's entirely plausible that he could live with her and his siblings but not know the identity of his father.
Tailkinker ★