Paddy is short for Pádraig (masculine), the Irish Gaelic name, which is the source of the ‘dd’ in St. Paddy’s Day.
Patty, on the other hand, is used to describe Patricia’s nickname (feminine), or a hamburger patty.
I saw ‘St. Patty’s Day’ written more times in newspapers, on business billboards and blogs, more times than I can count this past St. Paddy’s Day and the days and weeks leading up to it.
FYI for St. Paddy’s Day in 2019, try to get it right. Do not drink before you write or put up your billboards, letter board stands, newsletters and whatnot.
There is a whole website dedicated to St. Paddy’s Day or St. Patty’s Day.
St. Paddy’s Day!
Don’t call it St. Patty’s Day!
Yes, you got my Irish up!