Its 'Day' as a Holy Feast was moved so as not to clash with Palm Sunday, or something tenuous like that. Its not simply because it is a weekday.
However, 17th March remains St Patrick's Day as far as most people are concerned, including myself. Ok?
Palm Sunday starts Holy Week so therefore until after Good Friday there shouldn't be any celebrating (I'm not RC but was told this by a friend who is) which is why it was brought forward to the Saturday.