Concept:
It is vital to distinguish between the date of "adoption" and the date of "commencement" of the Indian Constitution. While the Constitution was ready and signed in late 1949, it only became fully operational a few months later.
Step 1: The Date of Adoption.
The Preamble of the Constitution explicitly states that the people of India, in their Constituent Assembly, did "adopt, enact and give to ourselves this Constitution" on the 26th day of November, 1949 (1). This day is now celebrated annually as Constitution Day (Samvidhan Divas).
Step 2: The Date of Commencement.
The 26th of January, 1950, is the date the Constitution came into force, marking India's transition to a Republic. This date was chosen specifically to commemorate the Declaration of Indian Independence (Purna Swaraj) of 1930.