I am not sure that 9/11 should be a holiday, but then again, other horrific events have served as the basis for holidays in the past.
"The 11th hour of the 11th day of the 11th month" so the saying goes, memorializes Armistice Day, which marked the end of World War I. Today we call it Veterans Day, which Kurt Vonnegut, a veteran himself, who was born on that day, decried (more than once) in his novels:
“Oh, it’s just so damn cheap, so damn typical.” I said, “This used to be a day in honor of the dead of World War One, but the living couldn’t keep their grubby hands off of it, wanted the glory of the dead for themselves. So typical, so typical. Any time anything of real dignity appears in this country, it’s torn to shreds and thrown to the mob.”