S&P BSE Sensex and NSE Nifty 50 indexes staged their biggest single-day gain of May powered by a rally in banking and financial heavyweights like HDFC Bank, ICICI Bank, HDFC, Axis Bank and Kotak Mahindra Bank. The Sensex rallied as much as 1,051 points and Nifty 50 index surpassed its important psychological level of 9,300. Earlier in the day, Sensex and Nifty flipped-flopped between gains and losses however strong buying interest in banking shares on account of short-covering a day ahead of futures and options expiry led to strong gains during the session analysts said.