According to The Smoking Gun, "an official tribute sign erected on the fence surrounding Ground Zero actually misstated the date of the terror attacks," giving the date as September 11, 2002. Eh, we were pretty much drunk for a year, so how are the city's graphic designers supposed to remember?
September 11, 2002 was actually a pivotal day in New York City — it was the day the media forced itself to stop writing "crisis sex" stories. By then, if the tabloids could be believed, the entire population of NYC had already had sex with 5.2 firefighters each.
Ground Zero Typo Caught After Two Years [The Smoking Gun]