We are well into March, and on Twitter the vigilant @paddynotpatty has already had his hands full with references to the impending "St. Patty's Day."
Do not draw his attention, for fear of looking a fool.
The diminutive form of Patrick, from the Irish Padraig, is Paddy. If you must be cute about March 17, call it St. Paddy's Day.
Patty is the diminutive form of Patricia.
Paddy is also a slang term for an Irishman, one that can give offense because of condescending, stereotypical associations. A police van, for example, is sometimes called a paddy wagon. The New Oxford American Dictionary speculates that that came about in the 1930s or so because many police officers in major Eastern cities were of Irish descent. I suspect that the term may be associated with the stereotype of an Irishman as someone who drinks up his weekly wages, becomes violent, and has to be carted away to jail to sleep it off. Your sense of the etymology of paddy wagon will depend on whether you think the term refers to the driver or the cargo. In any case, steer clear of it; you don't want to get anyone's Irish up.
The saint's feast day, in this country, has become a boozy excess involving people pretending to be Irish. St. Patrick himself was not Irish, having been born in Roman Britain. So no harm and no foul if you choose to be honorary Irish on the grand day as you lift a pint of Smithwick's or Guinness to your lips. Slainte.
I don't plan to chime in on @paddynotpatty's Twitter campaign this year. When I did point out that St. Patty is wrong. I got responses along the lines of "Well, this is America." If American means to you "inclined to express belligerent ignorance," go ahead; it's a free country.
But you'll still be an eejit.