We’ve got St. Patrick’s Day, which is, I presume, a day in which to appreciate and celebrate Irish history and culture and all the wonderful things the Irish have contributed to civilization. I have no problem with that.
But why the Irish? Why don’t we have a day to celebrate, say, French culture? Or Japanese? Or Hawaiian? Why only Irish?
Immigration, me boyo. Sheer vast waves of immigration and the fact that they haven’t ever declared war on us. Else we’d all know when Steuben Day is.
September 17.
Because the Irish were/are a substantial, discriminated-against minority in the US. At one time, at least. Signs were common that said “No Irish Allowed.” St. Patrick’s Day was a way to celebrate their heritage. It caught on.
A parallel is the celebration of Cinco de Mayo by Mexicans in the US. The holiday is virtually unknown ssouth of the border, but it is a way for Mexicans and those of Mexican descent in this country to celebrate their roots.
For the second part of your question: the French were never a discriminated-against minority, the Japanese were never here in any numbers, ditto the Hawaiians.
It was a US tradition, a way for large numbers of Irish immigrants to celebrate their heritage. Here’s a nice little summary from Slate. And here’s a longer article discussing the rise in popularity of “traditional” Irish pubs and how, even in Ireland, St.Patricks Day has changed from a small family holiday into an agressively marketed “Mardi Gras” type party.
It probably helped that it coincides with Evacuation Day in Boston, which celebrates the end of the Siege of Boston with the withdrawal of British troops (this became a holiday around the same time that the Irish began to gain a lot of political power in Boston, incidentally).
Plus, I mean, comon! Green beer! WIN!
Evacuation Day was picked as a holiday so the Irish could have St Patrick’s Day off, like the Italians had Columbus Day. It’s not a coincidence, it’s the whole reason we have Evacuation Day.
Well, eveyone is wrong- but right. What you all have is a reason why the Holiday started. But honestly, it should be a minor local holiday. The *real * reason why we celebrate it is that it is a good adult holiday to go drink. If it wasn’t for that, it’d just be another line on most dudes calendars, with a parade in a few cities with a large Irish population.
Great cites, btw tremorviolet. 
Exactly my thought. St. Patty’s Day is widespread because the Irish made their day fun and welcomed everyone to participate (well except for the openly gay but that is a subject for GD or the Pit). Have a parade and then go drinking and overeating. All you need to do to fit in is wear green. It is a fun Holiday. Even coincides with March Madness for an extra bonus of getting together.
Jim {I am 1/8[sup]th[/sup] Irish except this weekend when I am all Irish at Heart}
Hey, I’m Irish, and you can get drunk with me whether you’re gay or straight. It’ll have to wait till next year when St. Paddy’s is on Saturday, though. I’m just too tired today.
Because we freakin’ ROCK!!
Obviously.
Please if you’re Irish then you can just go drinking tomorrow! 
Even MORE ironic in my book is that St. Pat’s day is supposed to be a Holy day.
You know, that whole bit about St. Patrick bringing Christianity to the Celtics?
Bastille Day.
King Kamehameha Day.