Every year on March 17, the people of the world congratulate Alec Baldwin's 17-year-old daughter Ireland on her new modeling contract by celebrating St. Patrick's Day. Unfortunately, in their attempts to observe St. Paddy's day, many people accidentally observe "St. Patty's Day."
"Patty" is a woman's name. The nickname used for a man named Patrick, for example, the man named Patrick who is credited with converting great swaths of Ireland to Christianity, is "Paddy," from the Irish Padraig.
Calling St. Patrick's Day "St. Patty's Day" is like referring to Christmas Eve as "Christie's Eve" or Hanukkah as "Helen's Festival of Lights."
How Did Padraig Become Patty?
Just as the process that transforms shamrocks into McFlurries is murky, so is it unclear exactly where or when Padraig's feast day became Patty's tea party. The confusion obviously has something to do with the fact that the Irish name Padraig is Anglicized Patrick. But the English nickname is "Pat," not "Paddy."
"Patty" is probably an American thing, like a McDonald's hamburger patty.
Isn't "Paddy" a Slur?
While it's true that "paddy" came into fashion as a slur against Irish people in the 19th century, it's also true that Paddy is just a regular old name still in use today. You'll have to go by intent on this one. Calling a person "a paddy" because he's Irish is offensive. Calling a person "Paddy" because his name is Paddy, is not. Calling St. Patrick "Paddy" might upset some people since he's a canonized saint and not just some guy you know.
If someone gets rankled by your use of "St. Paddy," revert back to "St. Patrick," which is more correct, not "St. Patty," which is less. (And don't get into a big fight on St. Patrick's Day. It's a happy day.)
I Still Want to Celebrate St. Patty's Day
That's great. You can do that too. Saint Patricia was born into a noble, possibly royal family (some sources describe her as the niece or granddaughter of Constantine the Great) in Constantinople (now Istanbul). When Patricia—"Patty," to you—was a teenager, she fled to Rome to become a Bride of Christ (nun). Later, she left for a pilgrimage to Jerusalem. Along the way, her ship was caught in a storm and she and her party were shipwrecked on a tiny island near Naples, where she established a chichi prayer community. She fell ill and died at age 21.
According to legend, over 500 years later, a knight paying his respects to Trish's remains decided he wanted a souvenir and plucked out one of her teeth, because obviously. Miraculously, blood started pouring out of the long-dead cavity—blood that nuns preserved in two glass vials. Today that blood is display at the San Gregorio Armeno Church in Naples, where it is said to turn back liquid every Tuesday morning at around 9:30 and every August 25th (Patricia's feast day).
All of which is to say: Celebrate St. Patty's Day on August 25th.
No, I Mean I Just Want to Call St. Patrick's Day "St. Patty's Day"
Well then you are just being willfully wrong. We have offered you knowledge and you have taken it, examined it, and deliberately rejected it. Cast it out like so many snakes from Ireland. (By the way: Ireland never had snakes. The "snakes" St. Patrick banished were probably the ones used in pagan symbolism, after he converted the druids to Christianity.)
If you're American, the words are even pronounced in the same way. We're just asking that you adopt the correct spelling, which, incidentally, is already phonetic.
So What Can I Call It?
You can call it "St. Patrick's Day." You can call it "St. Paddy's Day." You can call it "St. Pat's." You can call it "Maewyn Succat's Day" (after Saint Patrick's birthname). You can call it "March 17th." You can even call it "Liberalia," an ancient Roman holiday also celebrated on March 17 is probably much closer to modern American St. Patrick's Day celebrations in spirit. (A procession of people carried a giant penis through the countryside, and everyone sang dirty songs and left food everywhere.)
Don't call it "St. Patty's Day".