If there had been celebrations like that in the U.S., they would have drawn instant attention and outrage. As John Hinderaker notes at Powerline, the Washington Post reported on Sep. 18, 2011 that “law enforcement authorities detained and questioned a number of people who were allegedly seen celebrating the attacks and holding tailgate-style parties on rooftops while they watched the devastation on the other side of the river.” That seems to provide at least some basis for Trump’s claim that celebrations happened.
However, an article on the same day in the New Jersey Star-Ledger reported: “Rumors of rooftop celebrations of the attack by Muslims here proved unfounded. So did reports circulating in Muslim communities that dozens of Arabs have been attacked in Jersey City.”
It was a time of fear and mutual suspicion, when all kinds of things seemed believable. But whatever celebrations happened in Jersey City on 9/11, it is safe to say they did not involve thousands of people–i.e. thousands of potential witnesses.