Impossible Astronaut Day

"The Impossible Astronaut" is the first episode of the sixth series of the British science fiction television series Doctor Who. Written by show runner Steven Moffat, and directed by Toby Haynes, the episode was first broadcast on 23 April 2011 in the United Kingdom, as well as the United States and Canada. The episode features alien time traveller the Doctor (Matt Smith) and his companions Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill), and is the first of a two-part story, which concluded with "Day of the Moon".

On April 23rd, 2011, the Doctor Who episode “The Impossible Astronaut” premiered, starting off series 6 with some mind-blowing plot twists and some new monsters, the Silence. The Silence are creatures that you can’t remember seeing the second you look away from them. To combat this unusual type of warfare, The Doctor, Amy, Rory, and River draw tally marks on their skin to remind them that they’ve had an encounter.
To celebrate the day, Whovians drew tally marks on their arms (some even drew on their face) and went about their normal day, pretending like they didn’t know where the tally marks came from. Basically, Whovians love having inside jokes that freak people out.