What led the 1000 points rally in the Sensex on 17 April? Was it because of the RBI package announced by the governor?

You are right, that was the trigger for the markets. In fact, the indices closed sharply higher after RBI announced measures aimed at easing financial stress stemming from the nationwide lockdown. The measures were essentially aimed at making life simpler for banks and NBFCs. Clearly banks led the rally and the Sensex settled 986 points or 3.2% higher at 31,588. Other rate sensitives like NBFCs, autos and realty also rallied. It may be recollected that RBI cut reverse repo rate by 25 bps to 3.75% to push banks to deploy excess funds toward lending. The top gainers on the index were stocks like Axis Bank, ICICI Bank, Maruti Suzuki and IndusInd Bank. Some of the software and FMCG stocks were among the losers. The A/D ratio was in the ratio of 23:7 on the Sensex while the VIX showed signs of easing.