According to the 9/11 Memorial & Museum:
Tribute in Light is a commemorative public art installation first presented six months after 9/11 and then every year thereafter, from dusk to dawn, on the night of September 11. It has become an iconic symbol that both honors those killed and celebrates the unbreakable spirit of New York….
WPIX Channel 11 News has a behind the scenes look at the 9/11 Tribute In Light testing
and you can see the Tribute In Light starting 6:30 tonight.