CBS has announced the details of 11th episode of the season, The Maternal Congruence, in which Christine Baranski returns as Leonard’s mother. The episode will air December 14th and will be the final original episode of the year and the decade.

“The Maternal Congruence” – A visit from Leonard’s mother delights Sheldon and horrifies Leonard, on THE BIG BANG THEORY, Monday, Dec. 14 (9:31-10:00 PM, ET/PT) on the CBS Television Network.
Source: http://the-big-bang-theory.com/