Pirates' Day

Pirate's Day is an annual world event that takes place on September 19, lasting for one day.

From Pirate's Day on the official site:
 * Avast, mateys! Commoners outfitted as scurvy pirates have appeared in all the world’s cities bringing rum news: the Dread Captain DeMeza and her crew have landed in Booty Bay and declared this day to be Pirates’ Day! If you’re brave enough to share her grog, you may have what it takes to become an honorary crew member for the day.

Details
First, talk to any pirate commoner to receive a costume to help you get into the spirit of Pirates' Day. Once appropriately attired, head down to Booty Bay to join in the fun. There, look for Dread Captain DeMeza and her crew, who have taken over the roof of the Booty Bay bank and auction house. By talking to Captain DeMeza, players become an honorary crew member and transform into a pirate of their race and gender for 12 hours. This costume buff may be clicked off but persists through combat, mounting, death, and so on but disappears if shapeshifted. Other highlights of the holiday include:

Also known as the authors of the book "Pirattitude", which teaches you how to be a pirate, John "Ol' Chumbucket" Baur and Mark "Cap'n Slappy" Summers.
 * A large crowd of pirate revelers ("Dread Crew") who dance, talk, eat, drink, laugh, and shoot fireworks. Ask what he thinks of the visitors.
 * The Tauren and his giant South Seas assault parrot,.
 * A trio of cannons and cannoneers who periodically fire out into the bay - at times, just barely missing the sails of the incoming transport. Those crazy corsairs.
 * An achievement awarded simply by speaking to DeMeza, The Captain's Booty!
 * and, two curiously-named pirates.
 * Lively pirate-themed music that plays upon the rooftop. Now can regularly be heard on the human-style transport ships sailing between the Eastern Kingdoms and Kalimdor.

Quests

 * You're Gonna Need A Bigger Boat!

Trivia

 * September 19th is the annual "", which is especially popular in online communities.