Wired has takent the opportunity to post A Reality Check - 9 Pirate Myths Examined with useful links embedded in the text, including one to an article from Slate, published June 5, 2007, that examines the origins of what we think of as pirate-speech. Both articles argue strongly that we're really celebrating Talk Like Robert Newton Day.
Want to really talk like a pirate? Of course, pirates today probably speak like everyone else does, only possibly more strongly. (I would rather not find out for sure.) But Christopher Bonanos, the author of the Slate article, provides instructions for speaking like a historic pirate:
...onboard speech was most likely underclass British sailor with extra curse words, augmented with a polyglot slang of French, Italian, Spanish, and Dutch picked up around the trade routes
Cheers, bon chance, and good [expletive deleted] celebrations!