This year is will be the 248th St. Patrick's Day Parade. While the parade honors Saint Patrick, the patron Saint of the Archdiocese of New York, it's become a popular celebration of anything remotely Irish, be it the heritage of a person or a beer.
The National Guard's Fighting 69th Infantry Regiment, which had been deployed in Afghanistan, will be leading the parade. This year, the parade's Grand Marshal is Michael Gibbons, the President Emeritus of the Ireland-U.S. Council; here's a PDF of the Line of March—Mayor Bloomberg, Police Commissioner Ray Kelly, Governor Paterson, plus other officials will be present.
At 11 a.m. tomorrow, the Saint Patrick's Day Parade will start at 44th Street and Fifth Avenue and will travel up Fifth Avenue to 86th Street, finishing around 4 or 5 p.m. Which means there are many street closings—besides the parade route, the parade formation area is 44th to 46th Streets from Vanderbilt Avenue to Avenue of the Americas and 47th to 48th Streets from Park Avenue to Avenue of the Americas. While the Central Park transverse roads will be open to traffic, the NYPD says, "Pedestrian crossing along the parade route will be permitted on 49th Street to 55th Street, 57th Street and 58th Street, 59th Street east bound only and 60th Street west bound only." So take mass transit—the LIRR and Metro-North are running extra trains
The MTA, however, wants to remind you that alcoholic beverages are not permitted on the trains, platforms or stations. The NYPD reiterate that and add that alcohol consumption during the parade is prohibited: "Persons found with alcohol may be subject to summons and/or arrest. Police Officers will confiscate alcoholic beverages brought to the parade, as well as any of the following locations: Port Authority Bus Terminal; Staten Island Ferry Terminals; Grand Central Station; Penn Station; Central Park; and any public street.