In case you’re skeptical, from Wikipedia (not always reliable but, in this case, the best we’ve got):
International Talk Like a Pirate Day is a parodic holiday invented in 1995 by two Americans, John Baur ("Ol' Chum Bucket") and Mark Summers ("Cap'n Slappy"), who proclaimed September 19 each year as the day when everyone in the world should talk like pirates. For example, instead of "hello," an observer of this holiday would greet his mates with "Ahoy, me hearty!" The date was selected because it is the birthday of Summers' ex-wife and would consequently be easy for him to remember.
Well, as long as there’s a good reason.