What’s the story with Patty

So I’ve been doing a bit of surfing, and what with St. Patrick’s day having just been, there are quite a few sites that mention it. So my question: when did it become Patty’s day?

Or is it just an Americanisation of Paddy’s Day? Coz I use paddy’s day, makes it sound less like a religous day to me, but Patty *shudder* I don’t like it. Makes me think of yer one in The Simpsons, Marge’s siter. Isn’t she called Patty?