Q: Exact date on which the first day of Diwali celebrated in 1972 and 1973
A: 1972: Main Diwali day = 14th Nov 1972. Chhoti Diwali/ Chaudas= 13th Nov 1972. Dhanteras was on = 12th Nov 1972. 1973: Main Diwali day = 3rd Nov 1973. Chhoti Diwali/Chaudas = 2nd Nov 1973. Dhanteras was on = 1st Nov 1973. These dates are from the Lunar Calendar. I hope this is what you need.