Hawaiian Leis

Hawaiian leis are known the world over as the symbol of aloha. A member of Captain Cook’s crew first recorded the custom of Hawaiians wearing flower garlands in 1779. Today, wearing Hawaii flower leis is more prevalent than ever, among both locals and visitors.
An Ancient Polynesian Custom
Early Polynesian voyagers who migrated to, and settled in the islands initially introduced the lei custom to the Hawaiian Islands. Leis were worn by these original Hawaiians to beautify themselves and distinguish themselves from others. The original Hawaiians excelled in the creation of permanent leis constructed of feathers, ivory, beads, and even teeth. Often, these leis were an emblem of the ali'i, or ruling class.
The maile lei was probably the most significant, because it was the lei of all people, all classes, and all occasions, but it was especially associated with the worship of the gods of hula. It was also used to signify a peace agreement between opposing chiefs.
A Modern Hawaiian Tradition
With the 20th century advent of tourism to the islands, the lei quickly became the symbol of Hawaii to millions of visitors worldwide. During the "Boat Days" of the early 1900s, lei vendors lined the pier at Aloha Tower to welcome arriving visitors and locals returning home. Departing visitors would throw their lei into the sea as their ship passed Diamond Head, hoping they would return to the islands again some day.
For the residents of Hawaii, the most popular occasions for lei giving are birthdays and graduations. It is not uncommon for a high school or college graduates to be seen wearing so many leis that they reach higher than the ears. It is common for locals to have a nut, seed or shell lei on hand to wear on special occasions.
Hawaii Flower Leis
Among the flowers used in modern Hawaiian leis are plumeria, lehua blossoms, and orchids, although maile leaves are extremely popular, and remain traditional adornments for hula dancers. Today, most visitors are only aware of the plumeria lei - a relative newcomer - widely available at airports and hotels.
There are very few rules when it comes to wearing Hawaiian lei. Anyone can wear one, anytime - there need be no special occasion. A lei is regarded as a celebration of one person's affection for another, so it's considered bad form to refuse one. The proper way to wear a lei is gently draped over the shoulders, hanging down both in front and in back. It is acceptable for one to purchase or make a lei for themselves.
Airport Lei Greeting
The airport lei greeting is now a standard experience in Hawaii. As tourism increased between Hawaii and the continental U.S., it became customary to present flower leis to those arriving or departing all of the islands by air, creating a cottage industry of lei construction.
These days, visitors experience a traditional welcome to the Islands with authentic Hawaiian leis right at their airline arrival gate. Vendors are pre-arranged to distribute Hawaii flower leis and assistance to baggage claim or directions to connecting island flights.
A friendly lei greeter stands just outside arrival gates at the Honolulu International Airport, Kahului, Lihue and Hilo airports, holding a welcome sign personalized with visitors' name on it. These vendors offer a Hawaiian-style reception with beautiful, fresh flower leis at an extremely reasonable price.
May Day is Lei Day in Hawaii
In Hawaii, May 1st is celebrated as Lei Day, rather than May Day, as the rest of the world does. This Festival of the Lei was first held on May 1, 1928, when a pair of local artists in Honolulu encouraged everyone to wear a lei. Festivities were held downtown with hula, music, lei making demonstrations, and exhibits and contests.
In 1929, Lei Day was made an official holiday in the Territory of Hawaii, a tradition that continues to the present. Lei Day, May 1st, remains a state holiday in Hawaii with festivities and celebrations all centered around the Hawaiian lei.

Resources:
http://www.hawaiiactive.com/activities/oahu-lei-greetings.html
http://www.myhawaii.net/activities/79-ma-lei-greetings.html