Short answers, pulled from the story.
India's Independence Day falls on the 15th of August each year. It commemorates the nation's independence from the United Kingdom on the 15th of August 1947, when the Indian Independence Act came into effect and transferred legislative sovereignty to the Indian Constituent Assembly.
The Constituent Assembly of India met for its fifth session at 11 pm on the 14th of August 1947, in the Constitution Hall in New Delhi. Independence was proclaimed at midnight, which is the transition point between the 14th and the 15th of August, so Nehru delivered the speech on the evening before the official date.
Between 250,000 and 1,000,000 people on both sides of the new borders died in the violence surrounding partition. The bloodshed was especially severe in Punjab, where the new border divided Sikh regions in half.
Gandhi did not attend the official independence ceremonies in New Delhi. He spent the 15th of August 1947 in Calcutta, observing a 24-hour fast and speaking to a crowd there to encourage peace between Hindus and Muslims.
Lord Mountbatten, the last viceroy of India, chose the 15th of August because it was the second anniversary of Japan's surrender in World War II. He advanced the transfer-of-power date from the June 1948 deadline announced by Prime Minister Clement Attlee on the 20th of February 1947.
On the 15th of August each year, the Prime Minister hoists the Indian national flag on the ramparts of the Red Fort in Delhi, delivers an address to the nation, and oversees a march past by the Indian Armed Forces and paramilitary forces. The tradition began on the 15th of August 1947, when Jawaharlal Nehru raised the flag above the Lahori Gate. The entire event is broadcast by Doordarshan, India's national broadcaster, and traditionally opens with the shehnai music of Ustad Bismillah Khan.