St. Patrick's Day , which celebrates the fifth-century missionary who converted many of the Irish to Christianity, was originally celebrated in Ireland with a meal of cabbage and bacon. Irish immigrants to America could not afford bacon so they substituted corned beef—and today, that's the dish most American celebrants enjoy on March 17.
Corned beef, typically made from brisket, is beef cured in salt brine. The term "corned" comes from the English term "corn," and refers to the coarse salt used for brining.
I've found Tony's Market  to be a great source for traditional corned beef—theirs is seasoned with pickling spices like mustard seed, red pepper, bay leaf, and peppercorn. If you've never made brisket at home, Tony's chef Mick Rosacci walks you through the super-simple process in this video . In short: put the brisket in a pan, cover it, cook for four hours, and then serve with cabbage, potatoes, and a mug of Irish stout. (Try the newly released Irish-style stout from Upslope Brewery  in Boulder.)
Across Town: Marczyk Fine Foods  has already begun corning its briskets for the holiday. The market brines the Niman Ranch  beef in water, salt, brown sugar, garlic and onion powders, crushed red pepper, and beet powder for two weeks.
Tony's, 950 Broadway, 720-880-4500 and five other locations 
Marczyk's, 770 E. 17th Ave., 303-894-9499 and 5100 E. Colfax Ave., 303-243-3355