https://zerodha.com/marketintel/Bulletin/569
February 19th, 2018, Monday is a settlement holiday. A settlement holiday is a day when trading is possible, but depositories are closed and the share credit to/debit from demat accounts does not take place. In light of the settlement holiday on the 19th, stocks bought on 16th February will not be available for selling on 19th February , i.e., BTST trades will not be possible.
Read more about settlement holidays here: https://zerodha.com/z-connect/traders-zone/holidays/settlement-holiday-and-its-impact