Pi Day was dreamed up 27 years ago to celebrate 3.14 on 3/14, but this year the geek dial is being turned up to 15 — as in 3/14/15. And this'll be the only year until 2115 that you can turn the dial up to the max, at 3/14/15, 9:26:53, an exact match for pi's first 10 digits.

As a matter of fact, there are two opportunities to celebrate the mysteries of 3.141592653 on Saturday.

"One of my former students sent me an email stating that he is going to celebrate the once-in-a-century Pi Day by eating pie both at 9:26:53 a.m. and 9:26:53 p.m.," Aziz Inan, an electrical engineering professor at the University of Portland, told NBC News in an email.

Did we say *two* opportunities? Strike that, and truncate the value at 3: At the Exploratorium in San Francisco, where physicist Larry Shaw organized the earliest known Pi Day celebration in 1988, they're going old school. The ceremonial, circular procession takes place at precisely 3/14 1:59 p.m., as usual — although the museum has a whole day's worth of events scheduled, including festivities in the Second Life virtual world.

NBC Universal is getting pi-happy as well: For example, NBC Learn, the company's educational arm, has put together an array of videos that explain mathematical concepts, including the relationship between pi and pizza pie. (NBCNews.com is a unit of NBCUniversal.)

## Pi? It's all relative

What is it about pi that tickles our geek glands? There's something mystical about the number, which is the ratio of a circle's circumference to its diameter. Even the ancients puzzled over its value; the Bible famously set it at 3. Today, it's been calculated out to more than 13 trillion digits with no end in sight.

As an abstract mathematical concept, pi is immutable. "Pi is what it is," said Brian Koberlein, an astrophysicist at the Rochester Institute of Technology.

But pi is squishier as a real-world measurement. In fact, Koberlein says Einstein's general theory of relativity dictates that pi can't be derived precisely by actually measuring a circle. "In a curved universe, you can never really get pi, because nothing is going to be perfectly flat," he told NBC News.

Circles never factor out to pi on a curved surface: For example, if you consider great circles on the surface of a sphere, the ratio of the circumference to the "diameter" (that is, a line on the surface connecting the opposite sides of the great circle) is exactly 2.

"You could use that as a measure of the curvature of space-time," Koberlein said. If Matthew McConaughey's character in "Interstellar" were to make measurements in the vicinity of the movie's gigantic black hole, he'd come up with a vastly different value for pi.

## Wacky or wise?

That relativistic twist in pi may sound wacky, but it's particularly apt this year — because 3/14 also marks Albert Einstein's birthday, and 2015 marks the 100th anniversary of his general relativity theory.

Wacky numerology is Inan's specialty: This year, the calendar-crazy professor and a colleague of his, Peter Osterberg, lay out a 14-step calculation that plays off the "hidden" connections between three-digit groupings in the decimal value of pi.

"Who knows, maybe the findings of this article will someday lead mathematicians to make a 'breakthrough' to prove or disprove, once and for all, if pi is a* normal* irrational number," they write.

Then there's the anti-pi movement, led by such mathematical luminaries as Vi Hart and Michael Hartl. In "The Tau Manifesto," Hartl lays out the case for getting rid of pi as a concept and replacing it with a circle constant known as tau, which is twice pi's value. University of Utah mathematician Bob Palais first set forth the idea in 2001, arguing that tau is easier to deal with.

Five years ago, Hartl turned the argument into a campaign — complete with its own Tau Day holiday on 6/28. "The Tau Manifesto project is making use of the ability to one-up pi," Hartl told NBC News.

Hartl fully acknowledges that 3/14 and 6/28 are both based on the vagaries of the U.S. tradition for calendar notation. The effect is totally spoiled if you think of Saturday's date as 2015/3/14, or 14/3/2015. Nevertheless, he sees the value of the, um, value.

"I don't think there's anything wrong in saying, 'OK, there's a visual reference to the value pi, or tau,' and using that as an opportunity to celebrate mathematics and broader geek culture," he said.

## What to do on Pi Day

Most math geeks feast on a pie of their choosing (fruit or pizza) on Pi Day. The International Business Times rounds up the places that are providing irrationally good deals on pies and other stuff to celebrate the holiday.

Looking for a good time? Loads of educational institutions are presenting Pi Day activities. There's a set of Pi K Fun Runs in the Chicago area. The Massachusetts Institute of Technology is even planning to announce their admissions decisions on 3/14/15 at 9:26 a.m. ET. The World Science Festival offers a Pi Day trivia quiz that hits the spot.

You can get additional pointers from last year's list of 3.14 ways to have fun on Pi Day. And check out these videos for a fresh batch of laughamatical moments:

If you happen to sleep through 3/14/15 9:26:53, mark these dates on your calendar:

- April 5 is when 3.14 months of the year have elapsed.
- Tau Day arrives on June 28, but you'll have to wait until 2031 for the once-in-a-century 6/28/31.
- For those who don't dig MM/DD/YY notation, Pi Approximation Day (also known as Casual Pi Day) rolls around on July 22. That date plays off the fact that the fraction 22/7 is approximately equal to pi.
- There's always next year: 3/14/16 is actually a closer six-digit approximation of pi's value.