D-Day
Overview

D-Day is a term often used in military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable
Variable (mathematics)
In mathematics, a variable is a value that may change within the scope of a given problem or set of operations. In contrast, a constant is a value that remains unchanged, though often unknown or undetermined. The concepts of constants and variables are fundamental to many areas of mathematics and...

, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours
Military designation of days and hours
The military designation of days and hours within the North Atlantic Treaty Organisation , is specified in AAP-6 , NATO Glossary of Terms and Definitions, and marked in what follows...

for similar terms. The initial D in D-Day has had various meanings in the past, while more recently it has obtained the connotation of "Day" itself, thereby creating the phrase "Day-Day", or "Day of Days".
Discussions
Encyclopedia
D-Day is a term often used in military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable
Variable (mathematics)
In mathematics, a variable is a value that may change within the scope of a given problem or set of operations. In contrast, a constant is a value that remains unchanged, though often unknown or undetermined. The concepts of constants and variables are fundamental to many areas of mathematics and...

, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours
Military designation of days and hours
The military designation of days and hours within the North Atlantic Treaty Organisation , is specified in AAP-6 , NATO Glossary of Terms and Definitions, and marked in what follows...

for similar terms. The initial D in D-Day has had various meanings in the past, while more recently it has obtained the connotation of "Day" itself, thereby creating the phrase "Day-Day", or "Day of Days". On the same principle, the equivalent terms in French
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

, Basque
Basque language
Basque is the ancestral language of the Basque people, who inhabit the Basque Country, a region spanning an area in northeastern Spain and southwestern France. It is spoken by 25.7% of Basques in all territories...

, Romanian
Romanian language
Romanian Romanian Romanian (or Daco-Romanian; obsolete spellings Rumanian, Roumanian; self-designation: română, limba română ("the Romanian language") or românește (lit. "in Romanian") is a Romance language spoken by around 24 to 28 million people, primarily in Romania and Moldova...

, Slovenian
Slovenian language
Slovene or Slovenian is a South Slavic language spoken by approximately 2.5 million speakers worldwide, the majority of whom live in Slovenia. It is the first language of about 1.85 million people and is one of the 23 official and working languages of the European Union...

and Swedish
Swedish language
Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

are , , , and Dagen D, and Tag X in Germany.

The best known D-Day is June 6, 1944 — the day of the Normandy landings — initiating the Western Allied effort to liberate mainland Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

from Nazi
Nazi Germany
Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...

occupation during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. However, many other invasions and operations had a designated D-Day, both before and after that operation.

The terms D-Day and H-Hour
Military designation of days and hours
The military designation of days and hours within the North Atlantic Treaty Organisation , is specified in AAP-6 , NATO Glossary of Terms and Definitions, and marked in what follows...

are used for the day and hour on which a combat attack or operation is to be initiated. They designate the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential. For a given operation, the same D-Day and H-Hour apply for all units participating in it.

When used in combination with numbers, and plus or minus signs, these terms indicate the point of time preceding or following a specific action. Thus, H−3 means 3 hours before H-Hour, and D+3 means 3 days after D-Day. (By extension, H+75 minutes is used for H-Hour plus 1 hour and 15 minutes.)

Planning papers for large-scale operations are made up in detail long before specific dates are set. Thus, orders are issued for the various steps to be carried out on the D-Day or H-Hour minus or plus a certain number of days, hours, or minutes. At the appropriate time, a subsequent order is issued that states the actual day and times.

In spacecraft launchings, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

utilizes the term 'T-Time' for the timing of the launch sequence down to the second (rather than M-Minute and S-Second), as in the expression "T minus 10 seconds and counting" for their countdown clock.

When referencing a local time zone, "Zulu" refers to Universal Co-ordinated Time (formerly Greenwich Mean Time). For other zones see list of military time zones.

## History

The earliest use of these terms by the U.S. Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

that the United States Army Center of Military History has been able to find was during World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. In Field Order Number 9, First Army, American Expeditionary Force
American Expeditionary Force
The American Expeditionary Forces or AEF were the United States Armed Forces sent to Europe in World War I. During the United States campaigns in World War I the AEF fought in France alongside British and French allied forces in the last year of the war, against Imperial German forces...

s, dated 7 September 1918: "The First Army will attack at H hour on D day with the object of forcing the evacuation of the St. Mihiel Salient
Battle of Saint-Mihiel
The Battle of Saint-Mihiel was a World War I battle fought between September 12–15, 1918, involving the American Expeditionary Force and 48,000 French troops under the command of U.S. general John J. Pershing against German positions...

."

D-Day for the invasion of Normandy by the Allies was originally set for June 5, 1944, but bad weather and heavy seas caused Gen. Dwight D Eisenhower
Dwight D. Eisenhower
Dwight David "Ike" Eisenhower was the 34th President of the United States, from 1953 until 1961. He was a five-star general in the United States Army...

to delay until June 6 and that date has been popularly referred to ever since by the short title "D-Day". Because of the connotation with the invasion of Normandy, planners of later military operations sometimes avoided the term to prevent confusion. For example, Douglas MacArthur
Douglas MacArthur
General of the Army Douglas MacArthur was an American general and field marshal of the Philippine Army. He was a Chief of Staff of the United States Army during the 1930s and played a prominent role in the Pacific theater during World War II. He received the Medal of Honor for his service in the...

's invasion of Leyte
Battle of Leyte
The Battle of Leyte in the Pacific campaign of World War II was the invasion and conquest of the island of Leyte in the Philippines by American and Filipino guerrilla forces under the command of General Douglas MacArthur, who fought against the Imperial Japanese Army in the Philippines led by...

began on "A-Day", and the invasion of Okinawa
Battle of Okinawa
The Battle of Okinawa, codenamed Operation Iceberg, was fought on the Ryukyu Islands of Okinawa and was the largest amphibious assault in the Pacific War of World War II. The 82-day-long battle lasted from early April until mid-June 1945...

began on "L-Day". The Allies' proposed invasions of Japan
Operation Downfall
Operation Downfall was the Allied plan for the invasion of Japan near the end of World War II. The operation was cancelled when Japan surrendered after the atomic bombing of Hiroshima and Nagasaki and the Soviet Union's declaration of war against Japan. The operation had two parts: Operation...

would have begun on "X-Day" (on Kyūshū
Kyushu
is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands....

, scheduled for November 1945) and "Y-Day" (on Honshū
Honshu
is the largest island of Japan. The nation's main island, it is south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Strait...

, scheduled for March 1946).