St. Patrick's Day is the Irish national holiday, celebrating the nation's patron St. Patrick who brought Christianity to Ireland. Traditions include drinking beer, participating in parades and wearing green. Anyone who's ever been a school kid knows that if you don't wear green on St. Patrick's Day, you get pinched. The pinching tradition came fairly late and is more associated with American St. Patrick's Day celebrations than Irish ones.

Wearing Green
Ireland is often known as the Emerald Isle, because of its lush green fields and hillsides. In addition, the Irish flag contains a green field, representing the Catholic roots of the country as well as its physical landscape. Because St. Patrick's Day takes place in March, on the cusp of spring, green also takes on a more general aspect of growth and rebirth. For all of these reasons, wearing green has become synonymous with celebrating St. Patrick's Day.

Coming to America
Americans first officially celebrated St. Patrick's Day in 1737, when the first recorded celebration took place in the city of Boston. New York's first St. Patrick's Day celebration took place 20 years later in 1737. These celebration became more prominent in the 19th century. Emboldened by America's defeat of Great Britain, the Irish began moving to America in increasingly large numbers. Those numbers only increased when Great Britain ended a ban on Irish immigration in 1827. St. Patrick's Day celebrations thus increased as more and more Irishmen arrived on American shores.

Leprechauns
The mischievous mythical leprechauns form another piece of the pinching tradition. According to the legends, leprechauns delighted in causing mischief, which included tripping, kicking or otherwise harming humans. Wearing green supposedly rendered the wearer invisible to leprechauns, and thus spared the wearer their attentions.

Warning or Punishment
Pinching someone who isn't wearing green serves one of two functions. Since there are no actual leprechauns, it can be viewed as "picking up the slack" by emulating the (mild) pain a leprechaun can inflict. A more benevolent interpretation of the practice holds it as a warning: a pinch reminds the pinchee to put on something green before a leprechaun finds him and works some greater piece of mischief.

Balance
Some traditions hold that if someone accidentally pinches another person who is actually wearing green, the pinchee gets to pinch the other person back 10 times. The reasons for this tradition are self-evident, serving as a counterbalance for those who abuse the pinching tradition and ensuring that people don't get pinched unless they truly wear no green. Since pinching on St. Patrick's Day is often the purveyance of elementary schoolchildren, this rule sees a lot of variation.