Why is it called Paddy's Day?

Patty is a woman's nickname—short for Patricia—and not short for Patrick. For the record, the correct short form is St. Paddy's Day. According to the Provisional Government of Paddy, not Patty, the D's in Paddy come from the Irish spelling, Pádraig. Visit their site to be chastised in a proper Irish manner.

Just a nickname for Patrick.

The same as yanks have thanksgiving

because drunk fools are hauled off in the paddy wagon.

It was named after the "Paddy Wagon," in which the revelers would be hauled away.

It isn't. It's called SAINT Patrick's Day, named for the patron saint of Ireland. Some people use "Paddy" as a nickname for Patrick, but many in Ireland whose ancestors lived through the cruel days of British occupation (and passed down their stories of woe) bristle at the word. They say it's similar to calling an African-American a "n-word."

Paddy is a nickname for Patrick.