March 16, 1949 marks the date when the Banking Regulation Act came into force, which was a watershed moment in Indian banking history as it brought comprehensive regulation of banking companies in India. This act gave RBI extensive powers over banking companies and laid the foundation for bank nationalization later. The other options are incorrect: RBI was established in 1935, nationalized in 1949 (Jan 1), and bank nationalization happened in 1969 and 1980.