AKA Impossible Astronaut Day or Tally Mark Day et al.
By the way, it’s not my aim to freak anyone out. I live in a place where Whovians are deeply closeted, so I am “coming out” in favor of one of the most life-affirming cultural endeavors I’ve ever been exposed to. Doctor Who is good for you. Now, eat your broccoli.
To quote myself, a suggestion: there is such a thing as washable markers. Using a Sharpie might indicate a more serious commitment than warranted.