When the date of independence was decided in June 3 plan and announced to public, there was an outrage among astrologers across the country because 15 Aug 1947 was an unfortunate & unholy date according to astrological calculations. Alternative dates were suggested but Lord Mountbatten was adamant on Aug 15 (since it was his lucky date). As a workaround, the astrologers suggested the midnight hour between Aug 14 and 15 due to the simple reason that the day according to English starts at 12 AM, but according to Hindu calendar, starts at sunrise.
The astrologers had insisted that the speech of acknowledgement of transfer of power be done within the 48 minutes window (referred to as Abhijeet Muhurta) which lasted between 24 minutes before and after 12:15am i.e between 11:51pm & 12:39am. Nehru had to deliver a speech only within that timeframe and an additional constraint was that the speech had to end by 12 AM, so that the holy conch (Shanka) be blown to herald the birth of a new nation at the stroke of midnight hour, and the rest is history.