On Sheldon's date board on Mar 20th it reads "Battle Star Gallactica" ended. Battlestar is ONE word, not two, and there is only one "l" in Galactica. These mistakes are often done to catch up the nerds watching who live for this kind of stuff. It's a joke.
Sheldon recites the quote "Luke, I am your father." This is an often misquoted line from Star Wars: Episode V - The Empire Strikes Back (1980) (it's "No, I am your father").
Sheldon claims that the best wedding date would have been May 19, 1996, when the Hale-Bopp comet was first visible with the naked eye, but claims it is no good since Jon Pertwee of Doctor Who (1963) fame died the same day. First of all, Pertwee died a day later; secondly, the Hale-Bopp comet is also infamous for leading to the mass suicide of members of the Heaven's Gate cult less than a year later. They believed that their souls would be teleported onto a spaceship that was supposedly flying behind the comet, which sounds like something Sheldon would not want to be associated with.