Season 9, Episode 17 - The Celebration Experimentation
Aired February 25, 2016
After more than nine years together, the gang finally celebrates Sheldon's birthday, surprising him with a special guest, on the 200th episode of The Big Bang Theory.
Season 11, Episode 11 - The Celebration Reverberation
Aired December 14, 2017
As Sheldon plans a birthday celebration for Amy, Howard is looking forward to Halley's first birthday. Meanwhile, Leonard has the holiday blues when he receives a Christmas letter from his brother detailing his many accomplishments.