Pi Day (March 14th, or 3/14) is a special day for Pastafarians, the followers of the Church of the Flying Spaghetti Monster (FSM).

This satirical religion began in Kansas in 2005, when Bobby Henderson, a physics graduate, sent a letter to the Kansas Board of Education. The letter parodied the arguments for intelligent design in schools. Henderson suggested that teaching intelligent design should also include the alternative theory that the FSM created the universe.
Pi Day predates the Pastafarians, originating in San Francisco in 1988.
While Kansas did not take up Henderson’s challenge, the idea became a social movement to take a more light-hearted view of religion. Pastafarians believe the FSM is invisible and undetectable, that pirates are the original Pastafarians.They don't solicit funds from their followers, have no hierarchy or physical places of worship, and have a Pasta Lord's Prayer.
Our Pasta, who “Arghh” in heaven, Swallowed be thy shame. Thy Midgit come. Thy Sauce be yum, On top, some grated Parmesan. Give us this day our garlic bread. And give us our cutlasses, As we swashbuckle, splice the main-brace and cuss. And lead us into temptation, But deliver us some Pizza. For thine are Meatballs, and the beer, and the strippers, for ever and ever. RAmen
It’s hard to make this up, but someone did.
The central creation myth is that an invisible and undetectable Flying Spaghetti Monster, often in the shape of a pie (thus its association with Pi Day), created the universe after drinking heavily. Pastafarianism has received some limited recognition as a “legitimate” religion (as opposed to the illegitimate ones?). Adherents purportedly number in the tens of thousands and are primarily in North America, western Europe, Australia, and New Zealand. Some Pastafarians may wear pasta strainers in official photos.
The idea of Pastafarianism came about when school boards were adding Biblical creation to science curricula. The Flying Spaghetti Monster is an argument that the philosophic burden of proof lies upon those who make unfalsifiable claims, not on those who reject them. Pastafarians have engaged in disputes with creationists, helping to dissuade local school boards from adopting new rules for teaching evolution as an addendum to Biblical creation.
Pi Day started out as something else….
It was to celebrate mathematics, observing the Pi constant (3.14 ad infinitum) and its value in everyday life. It is also to make math more fun (I do not know how) and the story of William Jones, the 17th Century formulator of pi, which is the ratio of the circumference of a circle to its diameter.
It also observes the birth of Albert Einstein in Ulm, Germany on 14 March 1879, who was a lousy mathematician (by some accounts) but a brilliant theorist. However, it is also the anniversary of the death of Stephen Hawking in 2018.