Question

give reason why our constitution completed on 26 November 1949 but enforced on 26 January or 1950

Answer 1

26 January was chosen as the Republic day because it was on this day in 1930 when Declaration of Indian Independence (Purna Swaraj) was proclaimed by the Indian National Congress as opposed to the Dominion status offered by British Regime.
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Answer 2

Here is your answer

The reason January 26 was picked as the day for the constitution to come into force is because in 1929, during the Lahore Session of the Indian National Congress, where Jawaharlal Nehru was elected president, a resolution was passed demanding complete independence for the first time.