The Maternal Congruence

 is the 11th episode of Season 3 of CBS sitcom "The Big Bang Theory", and it's second (and last, up to date) christmas-themed episode. It aired on December 14th, 2009.

Synopsis
A visit from Leonard's mother, Dr. Beverly Hofstadter (Christine Baranski), delights Sheldon and horrifies Leonard. Penny is upset because Leonard didn't mention his mother's visit to her, but that is nothing compared to when Leonard learns that Sheldon knows more about his own mother than Leonard does.