Long before September 11, 2001, September 11 marked a less grim date: new year in the Coptic calendar. This is sometimes referred to as "Egyptian New Year" because the Coptic calendar, being solar, has long been used by Muslim Egyptians for agricultural purposes; the ancient Egyptian new year began after the height of the Nile flood, usually in August; the later Coptic calendar descends from the ancient one, and the first of the month of the Coptic month of Tut today coincides with September 11 in the Gregorian calendar. (And the month of Tut echoes the ancient Egyptian name Thoth, for the god of the same name.) Today marks the first day of the Year of the Martyrs 1730 (the Coptic calendar dates from the persecutions of Diocletian, not from the birth of Christ).
For unclear reasons, Coptic New Year is known as Neirouz, which seems to echo the Persian Nowruz which is in the spring. Some think an ancient "feast of the rivers" (Ni-Yarouou) somehow was conflated after the Arab conquest with the Persian word. Whatever the origin, a happy New Year to Copts (and those living by the Egyptian agricultural year).
It is also the Ethiopian New Year, by the way.