It’s the great day of 23rd march. All Pakistanis celebrate it as a reminder to Lahore resolution, which was passed on 23rd March, 1940.
Lahore resolution was presented by Tiger of Bengal, A.K. Fazlul Huq (sher-e-bangla).
An excerpt from Wikipedia says:
The Lahore Resolution
The Original text of Lahore resolution is as follows:
The Lahore Resolution
Resolved at the Lahore Session of All-India Muslim League
held on 22nd-24th March, 1940
(1) While approving and endorsing the action taken by the Council and the Working Committee of the All-India Muslim League indicated in their resolutions dated the 27th of August, 17th and 18th of September and 22nd of October, 1939, and 3rd February 1940 on the constitutional issue, this session of the All-India Muslim League emphatically reiterates that the scheme of federation embodied in the Government of India Act, 1935, is totally unsuited to, and unworkable in the peculiar conditions of this country and is altogether unacceptable to Muslim India.
(2) It further records its emphatic view that while the declaration dated the 18th of October, 1939 made by the Viceroy on behalf of His Majesty’s Government is reassuring in so far as it declares that the policy and plan on which the Government of India Act 1935, is based will be reconsidered in consultation with the various parties, interests and communities in India, Muslim India will not be satisfied unless the whole constitutional plan is reconsidered de novo and that no revised plan would be acceptable to the Muslims unless it is framed with their approval and consent.
(3) Resolved that it is the considered view of this session of the All India Muslim League that no constitutional plan would be workable in this country of acceptable to Muslims unless it is designed on the following basic principle, namely that geographically contiguous units are demarcated into regions which should be so constituted, with such territorial readjustments as may be necessary, that the areas in which the Muslims are numerically in a majority as in the North-Western and Eastern Zones of India, should be grouped to constitute “Independent States” in which the constituent units shall be autonomous and soverign.
That adequate, effective and mandatory safeguards should be specifically provided in the constitution for minorities in these units and in these regions for the protection of their religious, cultural, economic, political, administrative and other rights and interests in consultation with them; and in other parts of India where the Mussalmans are in a minority, adequate, effective and mandatory safeguards shall be specially provided in the constitution for them and other minorities for the protection of their religious, cultural, economic, political, administrative and other rights and interests in consultation with them.
This Session further authorises the working committee to frame a scheme of constitution in accordance with these basic principles, providing for the assumption finally by the respective regions of all powers such as defence, external, affairs, communications, customs and such other matters as may be necessary.
The Delhi Resolution
The Pakistan resolution of March 1940 was further clarified by a resolution unanimously passed by the Muslim League Convention of elected M.L.A.s of the Centre and from the Provincesheld, under the Presidentship of the Quaid-i-Azam at Delhi on April 9 1946. Among other things the said resolution specified that:
The zones comprising, Bengal and Assam in the North-East and Punjab, North-West Frontier Province, Sind and Baluchistan in the North-West of India, namely Pakistan Zones, where the Muslims are in a dominant majority, be constituted into a soverign independent state and that an unequivocal undertaking be given to implement the establishment of Pakistan without delay.
Hopefully this text would be helpful in knowing more about what exactly happened between India, Pakistan, Bangladesh (now) and British crown on 23rd March 1940.