Few subjects seem more dispassionate (OK, dull to some) than mathematics. Sitting in an exam room struggling to remember the formula for the area of a circle hardly inspires visions of fun and games, much less mystic revelation.

And yet, this Saturday, thousands of people around the world will unite to sing songs, recite poetry, perform bizarre rites and eat ritual food in honor of their favorite number.

The number is pi, 3.1415926535 . . . ad infinitum. It's the number you get when you divide the circumference of a circle by its diameter, and it can't be expressed as a fraction. It goes on forever.

Such so-called irrational numbers seemed so offensive when they were first discovered by the early Greeks, according to some accounts, that people were actually murdered for letting out the secret of their existence.

Pi day ceremonies, appropriately enough, take place on March 14, at 1:59 p.m. (That's the third month, the 14th day . . . and so on.)

The variety of celebratory modes is almost as long as pi itself, with dozens of Web sites devoted to the number's devotees--or piets, as they are called. Posted, along with formulas for calculating pi, are pi carols, poems and other utterly useless bits of pi trivia for honoring the day. There's even a Web site dedicated to the celebration of "pi approximation day," which takes place on July 22: 22/7 is the fraction that comes closest to the real value of pi. Some people even celebrate 2 pi day, June 28 (6/28).

The high holy day, however, is March 14. At the Exploratorium science museum in San Francisco, the faithful usually gather around the "pi shrine," a small brass plate (a pi plate, you might call it) engraved with pi to a hundred digits along with other obscure mystical symbols.

"We circumambulate the shrine 3.14 times," said Exploratorium scientist Ron Hipschman. They don't measure the distance exactly, however. "A little more than 3 is good enough for us," he said. "After all, pi is approximate."

Since pi goes on forever, you can't ever really pin down its exact value to the last decimal point. There is no last decimal point.

There is "all kinds of pi stuff" that goes along with the celebration, said Hipschman. Last year, someone composed music based on the number pi, he said. People add more beads to a ritual pi string--now up to 1,600 digits of pi, each digit represented by a colored bead signifying a number from 0 through 9.

And of course, they eat pie. All kinds of pie, including chocolate, apple, peach. "Sometimes we have pie fights," said Hipschman, "but we try to avoid that. It's kind of messy."

What is it about pi that makes it so much more than just a number?

For one thing, pi day is Albert Einstein's birthday. But that's just a coincidence that adds icing to the cake, like the fact that the first 144 digits of pi add up to 666, which some people equate with the devil.

Certainly, knowing the value of pi to 51 billion digits, as recently calculated on high-powered computers, is not particularly useful. With only 47 digits of pi, one can calculate the circumference of a circle around the visible universe to a resolution smaller than a subatomic particle.

Yet, writes William L. Schaff in his book, "Nature and History of Pi," "probably no symbol in mathematics has evoked as much mystery, romanticism, misconception and human interest as the number pi."

Schaff, along with dozens of other pi aficionados, is quoted in a new book devoted to pi, and published just in time for pi's special day. Titled "The Joy of Pi," by computer expert David Blatner, it's a feast of pi facts and fancies, just the thing for people who can't seem to get their fill of pi.

Blatner tells us that calculating pi does have uses. For example, such calculations are a good way to test the speed and accuracy of cutting-edge computers. "In fact, whenever individuals have attempted to break the world's record," he writes, "they have uncovered deep underlying flaws in their hardware or software that would have been almost impossible to identify any other way."

Finding yet another digit of pi is also a way of establishing computer bragging rights, he says--a race of "my computer is bigger than your computer."

However, even knowing 51 billion digits of pi will not help you find the 51-billion-and-first digit. Pi follows no perceptible pattern, which is probably why it enthralls scientists so.

Ultimately, pi's appeal is probably more philosophical than practical. Pi has fuzzy edges, like clouds or rainbows. It's not only irrational, it's also transcendental--meaning that it can't be expressed as an algebraic equation.

It is unfathomable and ungraspable. You can write it out, and there will always be more. Humans can't help but find that unsettling. Pi reminds us, says Blatner, of "the limits of our own comprehension."