Certain days of the year are celebrated or remembered for their political significance. There's USA Independence Day, Bastille Day, attack on Pearl Habour, Battle of Trafalgar, D-Day, etc.
But one doesn't refer to these historical moments by their dates, as 7/4, 7/14, 12/7, 10/21 or 6/6. They are mentioned by the significance of their events, for example, Bastille Day as in the storming of the Bastille, or the invasion of Europe against the Nazi occupiers designated by military planners as D-Day.
Why then do we have 9/11 and now 7/7 for the terrorist attacks on respectively New York's World Trade Centre's Twin Towers and London's Tube Stations? Why not refer to them as 'Attack on Twin Towers' (or WTC) or 'London Tube Bombings'? Why the use of numerals to indicate the dates of occurrence?
Well, we can explain the designation of one of the answers, namely 7/7 for the London Tube Bombings. Whoever started it followed the American example, either because it seemed logical at that time to 'standardise' reference to such terrorist attacks by their dates of occurrence, or to deliberately confer a horrendous image as that evoked by the Twin Tower collapses.
Wikipedia has this to say about 9/11:
"The attacks are often referred to simply as September 11 or 9/11. The latter is from the U.S. style for writing short dates, and is pronounced 'nine-eleven.' Within the United States, the typographic styling of the 9/11 designation alludes to 9-1-1 (also written 911, pronounced 'nine-one-one' in either case), the emergency telephone number used by the U.S. and Canada."
"Furthermore, the two 'ones' in 9/11 are seen by some as being representative of the two towers of the World Trade Centers. For these symbolic reasons and for convenience, 9/11 has become a common domestic term for the attacks."
"There was some initial speculation that the correspondence between 911 and the date 9/11 as mentioned above was intentional, to communicate something along the lines 'Starting now, life in America is about emergencies rather than ease.'"
But it doesn't explain, for example, why the Japanese sneak attack on Pearl Harbour has never been referred to as 12/7 or twelve-seven.
Personally I suspect that the origin of the numerals (of dates) to refer to momentous occasions, whether joyous or tragic, can be traced to Chinese traditional practice. Chinese are very fond of referring to such significant occasions by their dates, starting with the month in number and followed by the date of the day.
Chinese refer to the celebration of the first republic of China as 10-10 or double ten, when Sun Yat Sun declared China a republic on 10 October 1911. Double ten of course carries an additional symbol of very 'ong', a Chinese word (Hokkien) meaning blazing luck or fortune.
One day I was with a group of friends discussing a particular event, which we naturally referred to by the traditional numerical coding. Maniam was puzzled by the numbers and asked me what the hell we were referring to. I explained the Chinese practice of employing such numerical coding for ease of reference. His eyes lit up feeling as if he had been inducted into the Secret Society of Handsome Chinese Cryptographers - Maniam spoke such beautiful Hokkien so he's always been thought of as a Hokkien Penangite.
Soon there was no stopping him as he referred to virually all events using such codings. I had to explain to him his birthday or mine, or for that matter, the first time he ever bonked anyone, didn't count as a momentous occasion and therefore shouldn't be referred to by such codings because no one would have any clue what he was talking about. He grudingly accepted that personal birthdays weren't historically momentous dates for society in general but was adamant the sacrifice of his virginity definitely was. Ah, well ...
And what was that event that got him interested in Chinese numerical coding of events in the first place?