The flag of Pakistan, the Parcham-e-Sitāra-o-Hilāl, was adopted on August 11, 1947, three days before independence.
From a party banner to a national flag
Syed Amir-uddin Kedwaii designed it, and the source material was close at hand: the flag of the All-India Muslim League, founded in 1906, which had spent decades arguing for a separate Muslim-majority state. The new flag drew directly on that banner, with one addition that mattered. A white stripe at the hoist stood for the religious minorities who would be citizens of the new country. The Constituent Assembly of Pakistan approved the design on August 11, 1947, during the partition of British India, and Pakistan became independent on August 14.
Green, white, and the crescent
The flag is a dark green field in a 2:3 ratio, with a white crescent moon and five-pointed star at the center of the green and the white stripe running down the hoist. Green is Islam and the Muslim majority. White is the minorities, and peace. The crescent stands for progress and the star for light and knowledge.
The crescent and star appear on the flags of other Muslim-majority states, Turkey and Azerbaijan among them, where they carry much the same associations of growth and guidance.
Flag days and protocol
The flag flies on government buildings and military installations, and it comes out in quantity on Pakistan Day and Independence Day. Rules cover how it is handled, displayed, and disposed of, and it is flown at half-mast for mourning. Schools and public institutions build ceremonies around it. The navy and the other service branches use ensigns that keep the crescent and star but arrange them differently.
References
Government of Pakistan - National Symbols
International Federation of Vexillological Associations
Encyclopedia Britannica - Pakistan: History and Cultural Context