International Talk Like a Pirate Day is a parody holiday type of thing thing, recognizable by the way that people celebrating it say "Arr" and "Yarr" occasionally over the course of the day. Wikipedia says it was created in 1995 by a couple of guys from Oregon in the immediate wake of a racquetball injury, which actually took place on June 6—but, since that's also the anniversary of D-Day, they decided to hold in on an ex-wife's birthday—September 19—instead.
The day is relevant to us right now because when it comes around this year, Sea of Thieves players can score Obsidian cannons by watching certain Twitch streams with their linked accounts. The guns are a nice enough piece, I suppose—a little dull compared to some, really—but the hook is that they'll be part of a set, along with the Obsidian figurehead, hull, sails, and flag that Rare gave away during a Twitch Rivals event in June. Those items aren't available for purchase from the Sea of Thieves shipwright, and I don't expect the cannon skin will be either, so if you want it, this is your chance.
To get your cannon skin, you'll need to link your Sea of Thieves and Twitch accounts, and then watch at least 30 minutes (in total, not necessarily consecutively) of streaming at twitch.tv/seaofthieves or on one of the participating channels. Once the clock has ticked over sufficiently, the cannon will be added to your account automatically, although you might have to wait a bit for it to show up: Rare said it could take up to 72 hours for the big gun to appear.
That's all there is to it—watch a piece, get a piece, and you don't even have to talk like a pirate if you don't want to. Full details are up at seaofthieves.com, and speaking of cannons, don't forget that you can fire your pets out of them if you want to.