India celebrates Independence Day on 15 August because British rule officially ended on 15 August 1947, when power was transferred to independent India.

The date was brought forward by Lord Louis Mountbatten, the last Viceroy of British India. He had originally been given a deadline to transfer power by June 1948, but amid growing political tensions and communal violence, the transfer was advanced to August 1947.

Mountbatten later explained that he selected 15 August because it was the second anniversary of Japan's surrender in World War II in 1945. He had been the Supreme Allied Commander of the South East Asia Command when Japan surrendered.

The Indian Independence Act 1947 established 15 August 1947 as the date on which British rule would end and the new Dominions of India and Pakistan would come into existence.

Interestingly, 15 August was not always India's Independence Day. Before independence, the Indian National Congress had observed 26 January as Independence Day from 1930 to commemorate the declaration of Purna Swaraj (complete independence). After India became a republic on 26 January 1950, that date became Republic Day instead.

In short: 15 August was chosen primarily because Mountbatten advanced the transfer of power to August 1947 and specifically selected 15 August, partly because it marked the second anniversary of Japan's surrender.