Question

why did the indian consitution come into effect on 26th jan. 1950?

Answer 1

to know this we have to look into history

i.e. in the year of 1930 , on 26th of January deceleration of indian independence  (Purna Swaraj) was proclaimed by the Indian National Congress

so
this date has large importance in indian freedom struggle

Hence 26th jan was selected
our Constitution was finished in the last month's of 1949

So next 26th jan
is 26 January 1950

and therefore this date is selected

Answer 2

here is your answer. .... before independence our freedom fighters like GANDHI ji..etc decided that the India is a free nation on 26 Jan 1930 only. ...
to memorise that day
when constitution was drafted on 26 Nov 1948 so it came to pass afterwards but they declared that day as Republic day on that special day.....