Depends on what you mean by celebrate.
Having a party doesnt mean they believe St. Patrick to be a saint. Some do, some dont and yet they celebrate it in whatever way they choose to.
I dont think most people who celebrate it frankly know or care why they do.
I got into a big scene at my parish that is largly Irish on St. Patricks day not that long ago. I had said that St. Patrick was born in Scotland and isnt it funny most people dont know that.
You should have seen the reaction!
I could not believe it!
The Priest had to step in to defend me for crying out loud! And he is Irish!
I dont like the day at my parish since that episode so I go elsewhere now.