Here's a PSA from the Dublin Airport: Don't call it St. Patty's Day. Also, March 17 should never be referred to as Patty's Day either.
You may, however, call it St. Paddy's Day, or Paddy's Day. Also acceptable are the traditional St. Patrick's Day and Patrick's Day.
In a fogra (notice, in Gaelic) posted to its Facebook page, the airport addressed what is apparently a pet peeve: the improper use of St. Patty's Day in the United States and Canada.
"Please share this simple message with your friends and relations in the United States and Canada," the fogra reads. "Using the power of your network, hopefully we can banish the scourge of St Patty once and for all."
So what's the problem with St. Patty's Day?
Patty is a nickname for Patricia, a woman's name, according to the website paddynotpatty.com. St. Patrick was, of course, a man.
Paddy is appropriate because it comes from Padraig, a variant of the name Patrick.
But if you really want to impress an Irishman, you need only say: Lá Fhéile Pádraig Sona Daoibh (Happy St. Patrick's Day!).