Pakistan had obtained its independence from the British Raj the 14th of August 1947. 23 March was originally supposed to commemorate the adoption of the first constitution of Pakistan and thus the declaration of Pakistan as a republic. However, Field Marshal Ayub Khan abrogated the constitution and declared martial law. Khan's regime, in order to justify celebrating the national day, changed it to commemorate the 1940 landmark, during which All-India Muslim League passed the Lahore Resolution which later cemented the formation of a new nation in the sub-continent as Pakistan, even though it did not actually mention Pakistan at all. The Muslim League annual conference was held from 22'24 March 1940 and the Lahore Resolution was passed on 23 March.
Woh Fasl-e-Gul Jissay Andesha-e-Zawwal Na Ho
Yahan Jo Phool Khillay Khila Rahay Sadiyon
Yahan Say Khizan Ko Guzarnay Ki Majaal Na Ho
Yahan Jo Sabza Ugay Hamesha Sabz Rahay
Aur Aisa Sabz Kay Jisski Koi Misaal Na Ho
Khuda Karay Kay Na Kham Ho Sir-e-Wiqar-e-Watan
Aur Iss Kay Husn Ko Tashweesh-e-Mah o Saal Na Ho
Har Aik Fard Ho Tahzeeb-o-Fun Ka Aouj-e-Kamaal
Koi Malool Na Ho, Koi Khasta Haal Na Ho
Khuda Karay Meray Aik Bhi Ham Watan Kay Liyay
Hayat Jurram Na Ho, Zindagi Wabaal Na Ho