Yes, my last name is McAlarney and, in case you're wondering: 1) it is Irish, and 2) that does not count as one of the seven things. It does, however, establish my cred as a quasi authority for the seven things, at least by marriage. So here goes:
1) The date. The holiday is celebrated on the likely death date of the venerable saint, as a religious holiday in most places. The parades and drinking festivities are an American addition.
2) The Shamrock. St. Patrick used the Shamrock, the distinctive tri-leaf plant, to explain the Trinity, the concept of a triune God, to Irish pagans. The Shamrock was also a traditional symbol of rebirth (as in the spring time) as well as a symbol of Irish nationalism.
3) The wearing of the blue? The original color associated with St Patrick is called St. Patrick blue. It is a sky blue color. Green became associated with the holiday at a later date. The distinctive blue is still associated with the order of St Patrick and it is the national color of the island.
4) Pinching. Tradition dictates that if you don't wear green on St. Patrick's day, you should be pinched. Supposedly you are being pinched for nonconformity or perhaps to turn the pinched spot a bruised green. The custom was allegedly begun by the "wee folk," or leprechauns, who have a reputation for crankiness perhaps because they are the cobblers to the fairies.
5) Snakes. legend has it that St. Patrick banished snakes from Ireland, driving them into the sea after they annoyed him during a 40-day fast. Some historians believe that the imagery of this legend alludes to St. Patrick banishing Druids from Ireland.
6) Irish Soda Bread. While probably an American invention or at least a corruption of the Irish original, it is not made with yeast, but with baking soda, hence the name. Traditionally too it includes raisins, or sultanas, as they are known across the pond, for sweetness.
7) Corned Beef and cabbage is not traditional Irish fare on the Day. Beef is not a common dish traditionally in Ireland. In the old country they'll be eating cabbage and bacon. Corned beef probably became associated with The Day around the turn of the century when Irish immigrants living on the lower east side of Manhattan borrowed the idea from Jewish neighbors.
There you have it. Seven facts about the Day, suitable for correcting a, shall we say Higgins, during a conversation over a pint of what else, Guinness.