I will add something here which most of the Pakistanis don't know unfortunately.
The 27th annual session of All India Muslim League was held from 22nd March to 24th March 1940 which was presided over by Quaid e Azam.
The historical Lahore resolution (which was later named as Pakistan resolution) was moved by A.K. Fazlul Haq, the then Chief Minister of Bengal, on 23rd March 1940.
This resolution was passed on the last day of the annual session i.e. 24th March 1940.
So, technically speaking Pakistan Day should be celebrated on 24th March and not on 23rd March.
And this date 23rd March was chosen as the first constitution of Pakistan was passed on 23rd March 1956 as on that day Pakistan got rid of its dominion status and became Islamic Republic of Pakistan (Islami Jamhooria Pakistan) under the rule of Sikandar Mirza. Before 1956, only 14th August was celebrated.
For the next two years, 23rd March was celebrated as Republic Day (Yom e Jamhooria). Then Ayub came into power and from 1959 onwards Republic Day was replaced by Pakistan Day.
To sum up what I just said, 24th March should be celebrated as Pakistan Day instead of 23rd March.
@Mangus Ortus Novem