Pakistan’s first independence day was celebrated on August 15 but it was advanced to August 14 from 1948.
One of the primary reasons is that on the morning of August 14, 1947 the last British Viceroy Lord Mountbatten “transferred the power” to Pakistan’s Governor-General designate Muhammad Ali Jinnah at the Constituent Assembly Building in Karachi. The Viceroy then flew to New Delhi the same afternoon.
The birth of Pakistan was announced at the midnight of 15th August as was the case with India. Jinnah was sworn in as Governor-General and Liaquat Ali Khan took oath as Prime Minister on August 15. However, a debate arose on the actual date of Pakistan’s birth and as the transfer of powers had been signed on August 14.
In 1947, 14 August also coincided with the 27th day of the Holy month of Ramadan. The government had taken it as an auspicious coincidence and this added to the argument that August 14 should be the day Pakistanis celebrate the birth of their nation.
Also, it cannot be ignored that Pakistan preferred to have an identity distinct from India. So, from 1948 onwards the state of Pakistan began to celebrate August 14 as its day of Independence.