We have Feb. 29th -- otherwise known as Leap Day -- every four years to make up for the fact that the Earth doesn't circle the sun in exactly 365 days. Without that occasional extra day thrown in, our entire calendar and its corresponding seasons would eventually be way off and we'd be celebrating Christmas during bikini weather.

But it isn't just a scientific thing. There's a lot of lore behind it, and it's observed and celebrated in a variety of ways.

Individuals born on Feb. 29th are called "leaplings" or "leapers," and as much fun as they might have claiming to age only 25% as fast as the rest of us, people of yore once thought that leapling babies would inevitably prove sickly and "hard to raise." Celebrities with leap day birthdays include motivational speaker Tony Robbins, singer Ja Rule and actor Dennis Farina -- so we'll let you do the math there.

Relationships are also affected by February's extra day. A centuries-old tradition dictates it's okay for women to propose marriage to men (instead of the other way around), and that as long as the ladies were wearing red petticoats when they popped the question, men who refused their proposals had to pony up a silk gown and a kiss.

But even if the pair becomes betrothed, they might want to avoid actually tying the knot during a Leap Year. An old Greek superstition says couples who do so will have bad luck -- in fact, to this day, one out of every five engaged couples in Greece avoids scheduling nuptials during Leap Years.

Superstitions and folklore aside, people from 56 countries are observing this year's Feb. 29th in a more practical way -- they've dubbed it Rare Disease Day and have put out a call for more research into illnesses with no known cure.