Mr. Gawne is the author of many books about World War II. His latest is: Finding Your Father's War: A Practical Guide to Researching and Understanding Service in the World War II U.S. Army (Casemate).

Every June the press carries stories about the invasion of Normandy in 1944. One of the most common questions raised is “what does the term D-Day mean and where does it come from?” The answer to what it means is readily known. They are used for the day and hour on which a combat attack or operation is to be initiated. However, the question of where the U.S. Army got the term has remained a mystery.

The First World War saw the start of massive operations involving the coordination of hundreds of thousands of troops, enormous logistics operations, and week long artillery barrages. The need for staffs to begin planning the details of an attack weeks and months before, led to a simple notation for the planned date and hour of the attack: D-day and H-Hour. Once the plans were distributed, all the commander had to do was announce the correct date and hour, and everyone would begin working in synch. It also helped keep the secrecy of an attack by not having the exact dates and times written on every set of orders.

An artillery barrage set to be fired at H-Hour minus one (H-1) would be understood to begin one hour before the official start of the attack. A brigade could be told their objective was to reach hill X by H-Hour plus three (H+3). If the attack was delayed, or moved up the brigade would still know that they had 3 hours after the start of the attack to take that hill. By not using exact dates and times these carefully planned attacks had flexibility.

The first use of the term “D-Day, H-Hour” by U.S. forces appears to be around 7 September 1918 when the term was used in the First Army Field Order Number 9 regarding the battle of St. Mihiel. However, working further backwards in A.E.F. records to the Battle of Cantigny, the First Division Field Order Number 18 from 22 May 1918 uses the term “D-Day, J-Jour.” The American staffs had clearly been using the French terminology in their planning.

Soon afterwards these terms change to “D-Day” and “J-Day” in American orders. One might hypothesize that random letters were being chosen to indicate the time of operation, but this was simply a case of a translator not understanding that the correct English translation of “J-Jour” would actually be “D-Day.”

But could the French have borrowed the term from the British? In WW1 the British Army used the designation of “Z-Day” and “Zero Hour” for the time of attack. The days leading up to an attack were called X-Day and Y-Day. Artillery Instruction Number 267 issued by the Australian Corps on 23 September 1918 to the US 27th and 30th Divisions, who were serving with the British forces, use the terms Z-Day and Zero Hour. Orders issued internally by the two American divisions for the same attack change these references from Z-Day to D-Day.

So it appears that the term “D-Day,” which has become synonymous with the invasion and liberation of France, originated with the French themselves.