Talking about gaining something in the past tense

How would one say India gained independence on the 15th January 1947

Would it be 

15 august 1947 ko India ko aazadi milli

15 अगस्ट को भारत को आजादी मिली
15 august 1947 India aazad hogaya tha
15 अगस्त भारत आजाद होगेया था

Please explain the reasoning behind this

Both the correct statements. 

15 अगस्तको भारत को आजादी मिली  India got/gained  independence on 15th Aug
15 अगस्त भारत आजाद हो गया (NOT गेया) था   India became independent on 15th Aug