Constitution Day, also known as Samvidhan Divas, is celebrated in India in honour of Dr.B.R. Ambedkar, known as the architect of the Indian constitution. The Government of India declared 26 November as Constitution Day. On this day in 1949, the Constituent Assembly of India adopted the Constitution of India, and it went into effect on 26 January 1950.

The Government of India declared 26 November as Constitution Day on 19 November 2015 by a gazette notification. The Prime Minister of India Narendra Modi made the declaration on 11 October 2015 on the occasion of laying the foundation stone of the Ambedkar memorial in Mumbai.

The year of 2015 was the 125th birth anniversary of Dr. Ambedkar, who had chaired the drafting committee of the Constituent Assembly and played a pivotal role in the drafting of the constitution. Previously this day was celebrated as Law Day. 26 November was chosen to spread the importance of the constitution and to spread thoughts and ideas of Dr. Ambedkar.

source: wikipediaConstitution_of_India