26 january 1950
Actually it was adopted on 26 Nov 1949 (as written in Preamble of Indian Constitution) and 26 Jan 1950 was date of its commencement. This day was chosen because this was the day in 1930 that Purna Swaraj day was celebrated, following the resolution of the Lahore Session (December 1929) of the INC.
Republic Day marks the adoption of the constitution of India and the transition of the country to a republic on January 26, 1950.
