Anzac Day is a national day of remembrance in Australia and New Zealand, originally commemorated by both countries on 25 April every year to honour the members of the Australian and New Zealand Army Corps (ANZAC) who fought at Gallipoli in the Ottoman Empire during World War I. It now more broadly commemorates all those who served and died in military operations for their countries. Anzac Day is also observed in the Cook Islands, Niue, Pitcairn, and Tonga. It is no longer observed as a national holiday in Papua New Guinea or Samoa.

Anzac Day marks the anniversary of the first major military action fought by Australian and New Zealand forces during the First World War. The acronym ANZAC stands for Australian and New Zealand Army Corps, whose soldiers were known as Anzacs. Anzac Day remains one of the most important national occasions of both Australia and New Zealand, a rare instance of two sovereign countries not only sharing the same remembrance day, but making reference to both countries in its name. When war broke out in 1914, Australia and New Zealand had been dominions of the British Empire for thirteen and seven years respectively.

The Australian and New Zealand Army Corps (ANZAC) was a First World War army corps of the Mediterranean Expeditionary Force that was formed in Egypt in 1915 and operated during the Battle of Gallipoli. General William Birdwood commanded the corps, which comprised troops from the First Australian Imperial Force and 1st New Zealand Expeditionary Force. The corps was disbanded in 1916 following the Allied evacuation of the Gallipoli peninsula and the formation of I Anzac Corps and II Anzac Corps.

Plans for the formation began in November 1914 while the first contingent of Australian and New Zealand troops were still in convoy bound for, as they thought, Europe. However, following the experiences of the Canadian Expeditionary Force encamped on Salisbury Plain, it was decided not to subject the Australians and New Zealanders to the English winter and so they were diverted to Egypt for training before moving on to the Western Front in France. The British Secretary of State for War, Horatio Kitchener, appointed General William Birdwood, an officer of the British Indian Army, to the command of the corps and he furnished most of the corps staff from the Indian Army as well. Birdwood arrived in Cairo on 21 December 1914 to assume command of the corps.

A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun, also known as daytime. The period of time measured from local noon to the following local noon is called a solar day.

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. The unit of measurement for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time. The word day may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?" Day also refers to the part of the day that is not night — also known as 'daytime'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night (see circadian rhythms).