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. The pride they took in that name endures to this day, and Anzac Day remains one of the most important national occasions of both Australia and New Zealand.

When war broke out in 1914, Australia had been a Federal Commonwealth for only thirteen years. In 1915, Australian and New Zealand soldiers formed part of an Allied expedition that set out to capture the Gallipoli Peninsula, under a plan by Winston Churchill to open the way to the Black Sea for the Allied navies. The objective was to capture Istanbul, capital of the Ottoman Empire and an ally of Germany. The ANZAC force landed at Gallipoli on 25 April, meeting fierce resistance from the Turkish defenders. What had been planned as a bold strike to knock Turkey out of the war quickly became a stale-mate, and the campaign dragged on for eight months. At the end of 1915, the Allied forces were evacuated after both sides had suffered heavy casualties and endured great hardships. Over 8,000 Australian and 2,700 New Zealand soldiers died. News of the landing at Gallipoli made a profound impact on Australians and New Zealanders at home and 25 April quickly became the day on which they remembered the sacrifice of those who had died in war.

Though the Gallipoli campaign failed in its military objectives of capturing Istanbul and knocking Turkey out of the war, the Australian and New Zealand troops' actions during the campaign bequeathed an intangible but powerful legacy. The creation of what became known as an "Anzac legend" became an important part of the national identity in both countries. This shaped the ways they viewed both their past and their future.

On 30 April 1915, when the first news of the landing reached New Zealand, a half-day holiday was declared and impromptu services were held. The following year a public holiday was gazetted on 5 April and services to commemorate were organised by the returned servicemen.

The date, 25 April, was officially named Anzac Day in 1916; in that year it was marked by a wide variety of ceremonies and services in Australia and New Zealand. From 1916 onwards, in both Australia and New Zealand, Anzac services were held on or about 25 April, mainly organised by returned servicemen and school children in cooperation with local authorities.

Anzac Day was gazetted as a public holiday in New Zealand in 1920, through the Anza, after lobbying by the Royal New Zealand Returned and Services' Association, the RSA. In Australia at the 1921 State Premiers' Conference, it was decided that Anzac Day would be observed on 25 April each year. However, it was not observed uniformly in all the States.

During the 1920s, Anzac Day became established as a National Day of Commemoration for the 60,000 Australians and 18,000 New Zealanders who died during the war. The first year in which all the States observed some form of public holiday together on Anzac Day was 1927. By the mid-1930s, all the rituals now associated with the day — dawn vigils, marches, memorial services, reunions, sly two-up games — became part of Australian Anzac Day culture. New Zealand commemorations also adopted many of these rituals, with the dawn service being introduced from Australia in 1939.

With the coming of the Second World War, Anzac Day became a day on which to commemorate the lives of Australians and New Zealanders lost in that war as well and in subsequent years, the meaning of the day has been further broadened to include those killed in all the military operations in which the countries have been involved.

Australians and New Zealanders recognise 25 April as a ceremonial occasion, to reflect on the futility of war, and to remember those who fought and lost their lives for their country. Commemorative services are held at dawn, the time of the original landing, mainly at war memorials in cities and towns across both nations. One of the traditions of Anzac Day is the 'gunfire breakfast' (coffee with rum added) which occurs shortly after many dawn ceremonies, and recalls the 'breakfast' taken by many soldiers before facing battle. Later in the day, ex-servicemen and women meet and join in marches through the major cities and many smaller centres.