St. Patrick's Day is fine, and St. Paddy's Day is an acceptable name at a push, but it's not wise to call it St. Patty's Day, unless you want to anger 4.5 million Irish people.
The reason behind the ire is that the name is simply wrong - 'Paddy', as a shortened form of 'Patrick', comes from the Irish name 'Padraig', and is an acceptable nickname for any Patricks in your life.
Patty, however, isn't short for anything, apart from maybe 'Patricia'. Unless you're talking about a burger, it shouldn't be used.
That's the thinking behind the No More Patty Google Chrome extension, created by Dublin-based creative agency In the Company of Huskies.
The extension can be installed in a few clicks, and automatically replaces every online mention of the "very wrong" 'Patty' with the "absolutely right" 'Paddy'.
Daniel Nugent, who works at the agency, told Joe.ie: "We Irish are hugely proud of the fact that St. Patrick's Day is celebrated the world over and lovingly known as 'Paddy's Day.'"
"So, over the coming days, we'll be spreading the name of Paddy across social media and hopefully sparking a movement to drive 'Patty' away once and for all, just like St. Patrick did to those snakes."
The agency found the incorrect 'Patty' was used around 1 billion times on Twitter during this period last year, and they're keen to stop it happening again.
Americans appear to be the worst offenders, so be sure to keep an eye out for any improper variations this 17 March. St. Patrick would be proud.