Impossible Astronaut Day is celebrated on April 23 by fans of the sci-fi series "Doctor Who." This day commemorates the airing of the episode "The Impossible Astronaut," which is part of the show's sixth season. The series, which first premiered in 1963, follows the adventures of the Doctor, a time-traveling alien who navigates through time and space in a time machine called the Tardis. Fans, known as Whovians, observe this day by drawing tally marks on their skin, referencing the episode's plot where characters must remember the alien monsters called the Silence by marking their bodies. As part of the celebration, fans humorously act as if they are unaware of the significance of the tally marks. The episode is notable for introducing new alien monsters and has become one of the most-watched episodes in the series.