Military designation of days and hours
The military designation of days and hours within the North Atlantic Treaty Organisation (NATO), is specified in AAP-6 (STANAG
STANAG is the NATO abbreviation for Standardization Agreement, which sets up processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO state ratifies a STANAG and implements it within their own...

 3680), NATO Glossary of Terms and Definitions, and marked (NATO) in what follows. Those entries marked (US) are specific to the U.S., and defined only in Joint Publication JP 1-02, Department of Defense Dictionary of Military and Associated Terms.

References to days (or hours) preceding or following a designated day (or hour) use a plus or minus sign and an Arabic numeral following the letter. For example, "D−3" is 3 days prior to D-day, "C+7" is 7 days after C-day, "H−2" is 2 hours before H-hour, and so forth. In less formal contexts, the time is usually spelled out, so that "D−3" becomes "D minus three" or "D minus 3".

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...

:was 20 October 1944, the day the Leyte Island Operation (the 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 .
B-Day:has no military designation.
C-Day:is short for "Commencement Day" which usually means when deployment
Military deployment
Military deployment is the movement of armed forces and their logistical support infrastructure around the world.-United States:The United States Military defines the term as follows:...

 for an operation
Military operation
Military operation is the coordinated military actions of a state in response to a developing situation. These actions are designed as a military plan to resolve the situation in the state's favor. Operations may be of combat or non-combat types, and are referred to by a code name for the purpose...

 commences. It is called "Commencement Day" because before deployment candy is usually passed out to G.I.s from charitable organizations. (US)
D-Day:is the unnamed day on which an operation commences or is due to commence. This may be the commencement of hostilities or any other operation. The most famous D-Day was June 6, 1944, when "Operation Overlord
Operation Overlord
Operation Overlord was the code name for the Battle of Normandy, the operation that launched the invasion of German-occupied western Europe during World War II by Allied forces. The operation commenced on 6 June 1944 with the Normandy landings...

" began. Contrary to popular belief, the "D" does not stand for any specific word - the most popular being disembark. (NATO). According to the "D" stands for "Day".
E-Day:is the unnamed day on which a NATO exercise commences. (NATO)
F-Hour:is the effective time of announcement by the U.S. Secretary of Defense to the Military Departments of a decision to mobilize Reserve units. (US)
G-Day:is the unnamed day on which an order, normally national, is given to deploy a unit. (NATO)
H-Hour:is the specific time at which an operation or exercise commences, or is due to commence (this term is used also as a reference for the designation of days/hours before or after the event). (NATO)
I-Day:is used informally within the U.S. military bureaucracy to variously designate the "Implementation Day" or the (Delivery Order) "Issuance Day".
J-Day:was used during both world wars to designate the day an assault occurred.
K-Day:is the unnamed day on which a convoy
A convoy is a group of vehicles, typically motor vehicles or ships, traveling together for mutual support and protection. Often, a convoy is organized with armed defensive support, though it may also be used in a non-military sense, for example when driving through remote areas.-Age of Sail:Naval...

 system is introduced or is due to be introduced on any particular convoy lane. (NATO)
L-Hour:is the specific time at which deployment for an operation commences. (US)
L-Day:(for "Landing Day") was 1 April 1945, the day Operation Iceberg (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 .
M-Day:is the day on which mobilization
Mobilization is the act of assembling and making both troops and supplies ready for war. The word mobilization was first used, in a military context, in order to describe the preparation of the Prussian army during the 1850s and 1860s. Mobilization theories and techniques have continuously changed...

 commences or is due to commence. (NATO)
N-Day:is the unnamed day an active duty unit is notified for deployment or redeployment. (US)
O-Day:is a Maritime Prepositioning Force (MPF, the vanguard of a Marine Air-Ground Task Force
Marine Air-Ground Task Force
The Marine Air-Ground Task Force is a term used by the United States Marine Corps to describe the principal organization for all missions across the range of military operations. MAGTFs are a balanced air-ground, combined arms task organization of Marine Corps forces under a single commander that...

, MAGTF) term designating the day the Maritime Propositioning Ship Squadron (MPSRON) off-load begins, or the continuous flow of the Fly-In Echelon (FIE) commences, whichever is later.
P-Day:is the expected date at which the rate of production of a consumable equals the rate at which the item is required by the Armed Forces. (US)
Q-Day:was 23 June 1945, the day of the dress rehearsal of the first atom bomb test
Trinity test
Trinity was the code name of the first test of a nuclear weapon. This test was conducted by the United States Army on July 16, 1945, in the Jornada del Muerto desert about 35 miles southeast of Socorro, New Mexico, at the new White Sands Proving Ground, which incorporated the Alamogordo Bombing...

 ; nowadays it is sometimes used informally to mean "Quality Day", or the first day of the calendar quarter.
R-Day:is the unnamed day on which redeployment of major combat, combat support, and combat service support forces begins in an operation. (US)
S-Day:is the unnamed day the President authorizes Selective Reserve callup (not more than 200,000 men). (US)
T-Day:is the effective day coincident with Presidential declaration of national emergency and authorization of partial mobilization (not more than 1,000,000 personnel exclusive of the 200,000 callup). (US)
U-Day:has no military designation.
V-Day:is sometimes used to designate "Victory Day", the day an operation successfully concludes.
V-E Day:("Victory in Europe") designates 8 May 1945, the date when the Allies formally celebrated the defeat of Nazi Germany
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...

V-J Day:("Victory over Japan") designates 14 August 1945, the date of Japan's unconditional surrender.
W-Day:is the effective day the President takes the adversary decision to prepare for war (unambiguous strategic warning). (US)
X-Day may refer to:* X-Day , a festival associated with the Church of the SubGenius* X-Day , a 2002 manga written by Setona Mizushiro* March 20, 2003 anti-war protest, a protests against the Iraq War...

:was 1 November 1945, the day Operation Downfall
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...

 (the invasion of Japan) was to begin . The term also generically means "attack day".
Y-Day:was 1 March 1946, the day Operation Coronet
Operation Coronet
During World War II, two operations in the Pacific theater were called Operation Coronet.* an early planning name for Operation Chronicle, which was executed in June 1943...

 (the invasion of Tokyo Plains) was to occur.
Z-Day:was 10 June 1945, the day the Australian Imperial Forces landed in Brunei Bay
Brunei Bay
Brunei Bay is the gateway to Brunei and Borneo. It is located 4°45'-5°02'N, 114°58'-115°10'E; east of Bandar Seri Begawan. The Brunei portion of the bay is in two sections separated by a finger of Sarawak about 6km wide at the coast...

 to liberate Brunei
Brunei , officially the State of Brunei Darussalam or the Nation of Brunei, the Abode of Peace , is a sovereign state located on the north coast of the island of Borneo, in Southeast Asia...

, part of Operation Oboe Six.
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