The memorial was first turned on at 6:55 p.m. on March 11, 2002 by Valerie Webb, a 12-year-old girl who lost her father, a Port Authority police officer, in the attacks. Mayor Bloomberg and Gov. Pataki were with Valerie when she flipped the switch.
For the 11th Anniversary of 9/11, the Tribute in Lights will be illuminated from dusk on September 11, 2012 until dawn on September 12, 2012.
It is visible for many miles, and it has been reportedly seen from as far away as Rockland County (which is about an hour north of New York City).
You can read more about the Tribute in Light on Wikipedia.
Want to see it for yourself? The Tribute in Light is best viewed from the waterfront outside of Manhattan, including Jersey City, the Brooklyn Bridge Promenade and Gantry Plaza State Park, though the Tribute in Light can be seen from many places in and around New York City.