This is a Public Service Announcement.
St. Patrick’s Day is two weeks off, and those who observe it might wish to get the terminology right.
Though the Irish in my genome is the deplorable Scotch-Irish Presbyterian form, I do know this: Do not refer casually to St. Patrick’s Day as St. Patty’s Day, or you will betray ignorance and synthetic Irishness.
The diminutive form of Patrick, an anglicized form of the Irish Padraig, is Paddy.
Patty is a girl’s name, and St. Padraig/Patrick was not a girl.
If you want to be cute and casual about the saint’s feast day, and assume an intimacy that may not exist, you can call March 17 St. Paddy’s Day.
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.
I have been publishing this caution for some years, along with @paddynotpatty on Twitter, and it often gets a response along the lines of “This is Amurica and we say what we like here.” I suspect that some of those responses come from people who drink light beer and are therefore not educable. So yes, this is America and we talk as we like. Just be advised that if you persist in referring to St. Patty’s Day, anyone genuinely Irish will take you for an eejit.
St. Patrick himself was a Brit. So no harm and no foul if you choose to be honorary Irish on the grand day as you lift a pint of Guinness to your lips. Slainte.