It was a blockbuster Friday on September 20 for the stock market when the finance minister gave it an early Diwali bonus. The Sensex saw its biggest gain in a decade, rising 1,921 points to close at 38,015. Investors and India Inc celebrated the reduction in effective corporate tax rate from 34.94% to 25.17%, which includes all cess and surcharges for domestic companies. That translates to 14-15% boost to post-tax corporate earnings.