Republic Day is a public holiday in India, when the country marks and celebrates the date on which the Constitution of India came into effect on 26 January 1950, replacing the Government of India Act 1935 as the governing document of India and thus, turning the nation into a newly formed republic. The day also marks the transition of India from an autonomous Commonwealth realm with British Monarch as nominal head of the Indian Dominion, to a fully sovereign republic in the Commonwealth of Nations with the President of India as the nominal head of the Indian Union.

The Constitution was adopted by the Indian Constituent Assembly on 26 November 1949 and came into effect on 26 January 1950 with a democratic government system, completing the country's transition towards becoming an independent republic. 26 January was chosen as the date for Republic day because it was on this day in 1930 when the Declaration of Indian Independence (Purna Swaraj) was proclaimed by the Indian National Congress in lieu of the Realm status as a Dominion later instated by the departing British Regime.

The Constitution of India (IAST: Bhāratīya Saṃvidhāna) is the supreme law of India. The document lays down the framework that demarcates fundamental political code, structure, procedures, powers, and duties of government institutions and sets out fundamental rights, directive principles, and the duties of citizens. It is the longest written national constitution in the world.It imparts constitutional supremacy (not parliamentary supremacy, since it was created by a constituent assembly rather than Parliament) and was adopted by its people with a declaration in its preamble. Parliament cannot override the constitution.

It was adopted by the Constituent Assembly of India on 26 November 1949 and became effective on 26 January 1950. The constitution replaced the Government of India Act 1935 as the country's fundamental governing document, and the Dominion of India became the Republic of India. To ensure constitutional autochthony, its framers repealed prior acts of the British parliament in Article 395. India celebrates its constitution on 26 January as Republic Day.The constitution declares India a sovereign, socialist, secular, and democratic republic, assures its citizens justice, equality and liberty, and endeavours to promote fraternity. The original 1950 constitution is preserved in a helium-filled case at the Parliament House in New Delhi. The words "secular" and "socialist" were added to the preamble by 42nd amendment act in 1976, during the Emergency.