On the 23rd of March, from Wikipedia:
The Lahore Resolution, commonly known as the Pakistan Resolution, was the National documentation and a formal political statement adopted by the All India Muslim League at the occasion of its three-day general session on 22-24 March 1940 that called for greater Muslim autonomy in British India. This has been largely interpreted as a demand for a separate Muslim state, Pakistan. The resolution was presented by A. K. Fazlul Huq.
Although the idea of founding the state was introduced by Allama Iqbal in 1930 and the name Pakistan had been proposed by Choudhary Rahmat Ali in 1934, Muhammad Ali Jinnah and other leaders had kept firm belief in Hindu–Muslim unity. However, the volatile political climate and religious hostilities gave the idea stronger backing.
More here. Adil Najam has a nice post at All Things Pakistan:
We, as Pakistanis, have not really been kind to the legacy of either man. We turned both into idols. And once we convinced ourselves that these were ’supermen’ we conveniently absolved ourselves of the responsibility to learn from – let alone emulate – either. We are fond of celebrating but incapable of incorporating either the actions of Mr. Jinnah or the thoughts of Mohammad Iqbal.