Why was 15th August chosen as the Independence day :

On 15 August 2021 India celebrated it's 75th Independence day, which got Independence from the British Rule in 1947 August 15. But, the main question is why was 15th August chosen as the Independence day?, The reason behind that is,

 Since 1930 the Indian National Congress Celebrated January 26 as India's "Independence day" as decided in 1929 Lahore session. In the 1929 Lahore Session, the then Congress President Jawaharlal Nehru gave the call for "Poorna Swaraj".

To decide the Transfer of Power the newly elected Labour Party headed by Prime minister Clement Atlee appointed Louis Mountbatten. Who arrived as the last Viceroy of India on 21st February 1947,asked him to transfer power to India by 30th June 1948.But as Louis Mountbatten arrived the situation was very tough to handle by the British government, So the British wanted to leaves India as soon as possible. 

We can understand the Situation from this quote by by Chakravarti Rajagopalachari, 

"If he (Viceroy Mountbatten) 

had waited till June 1948,

There would have been no 

Power left to transfer". 

On 4th July 1947 Indian Independence Bill was introduced in the British House of commons, following inputs given by Viceroy Mountbatten. The Bill was passed fortnight and the Royal Assent was given on 18th July 1947. This laid down August 15th as the date for the transfer of power from the United Kingdom to India.

About Choosing the Date:

Mountbatten said "The date I chose came out of the blue. I chose it in reply to a question. I was determined to show I was master of the whole event. When they asked had we set a date, I knew it had to be soon. I thought it had to be about August or September and I then went out to the 15th August ".

Specifically, He chosen this date because it was the second anniversary of japan's surrender, and Louis Mountbatten was the Commander of Asian Naval Force at that time. So as to mark his authority he choose 15th August as the date for Transfer of Power.