India has been fighting against British colonial rule since 1757. By the 1920s, Indian National Congress emerged as the most prominent voice against foreign rule. An umbrella organization of a sort, it tried to win concessions through negotiations.
The demand till 1929 was of a dominion status under the English crown. At the Lahore Session in December 1929, under the Presidentship of Pt. Jawaharlal Nehru a resolution for Purna Swaraj (complete independence) was passed.
Complete independence was declared as the goal of Congress. Congress Working Committee took oath on the Purna Swaraj Resolution at the midnight of 31 December 1929. Afterward, Nehru hoisted a tricolor flag. It was decided that on 26 January, Indians would celebrate Independence Day.
On 26 January 1930, people across the country came out with tricolor national flags, took out a procession, and took pledges on the Purna Swaraj resolution publically. From 1930 till 1947, 26 January was celebrated as the independence day by a majority of Indians.
When India attained independence on 15 August 1947 and the constitution was completed on 26 November 1949, the Indian leadership wanted that 26 January should remain an important date. So, it was decided that the constitution would be implemented on 26 January 1950.
Thus making this date the Republic Day of India when every year the national flag would be hoisted by the head of the state in continuation of a practice established by the Congress leadership between 1930 and 1947.