Today, June 6, is perhaps best remembered for Operation Neptune which formed a part of a greater plan called Operation Overlord and specifically were the allied landings across Normandy. They would in turn led to the liberation and restoration of the French Republic and later, victory to the allies in the Second World War.
D-Day as noted by the United States Department of Defense in their "Dictionary of Military and Associated Terms" is an:
unnamed day on which operations commence or are scheduled to commence
This suggests something about 6th June 1944 that perhaps isn't immediately obvious. The allied invasion of France was headed by General Dwight D. Eisenhower, in his role as Supreme Commander of the Allied Forces in Europe. Naturally, this means that he was always going to use his existing terminology when planning the invasion. Also, since Montgomery, Bradley, Ramsay, Leigh-Mallory and Smith it also explains why they too would also use that same terminology.
Further to this, days like W-Day which is used as the day that the President of the United States makes an adversary decision to prepare for war, C-Day which is the day that a deployment operation commences or M-Day which is the day of mobilization, would also form part of official communications when it came to planning operations.
I supposed technically W-Day for the United States in the Second World War was the 8th of December 1941, which is the day after the "date which will live in infamy". W-Day would have been the date of formal declaration of war.
More formally though, apart from D-Day at least four more days were noted as lettered days:
A-Day - 20th Oct 1944 - Invasion of the Gulf of Leyte - Operation King Two
L-Day - 1st Apr 1945 - Invasion of Okinawa - Operation Iceberg
Z-Day - 10th Jun 1945 - Invasion of Borneo - Operation Oboe Six
Q-Day - 23rd Jun 1945 - Trinity Test dress rehersal for the Atomic Bomb - Manhattan Project
Two lettered days never took place:
X-Day - 1st Nov 1945 - Invasion of Kyushu - Operation Olympic
Y-Day - 1st Mar 1946 - Invasion of Honshu - Operation Coronet
I have it on somewhat second-hand information that the standard notation relative to a lettered day is the letter plus or minus the number of days. When "undoubtedly the most complicated and difficult operation undertaken" (as noted by Churchill) finally ended on 25th Aug 1944, I suppose that it would have been noted as D+80.
Of all these lettered days, one of the most important is conspicuous by its absence; that of course being the date when the guns fall silent and hostilities cease; those dates can never be written in advance by generals. Those dates are the most looked forward to though and generals and officials who make up plans for lettered days and lettered hours, would do well to remember that.