Friday the 13th

"What is six times seven" is posed as the question to the answer to life, the universe and everything (42). This is true in base 13, although this was merely coincidence according to author Douglas Adams

Every book in A Series of Unfortunate Events has 13 chapters, and it has been reported that there will be 13 books in the series when it is finished.

The 13 Ghosts of Scooby Doo, an animated TV series where the venerable cartoon dog finds and opens a chest containing 13 of the most powerful ghouls in the world, and is charged with putting them back.

..and Scooby got all of the ghosts back, just in case you were wondering.