read about St. Patrick's Day

I have a little trouble realizing why we, in the United States, should celebrate a holiday from some other country. I guess we do kind of celebrate others, like Cinco de Mayo but that is mainly just the Latinos who keep that one. Valentine's Day has appealed to citizens of the United States since it seems to be romantic, but we don't think of it as a celebration from another country. We accept that St. Patrick's day is Irish though, so I am still wondering, why do we celebrate it with parties, the wearing of the green, etc.