Not true! This is were the 20 days number comes from. However if my memory is correct there maybe 1 to 3 days that you will be unable to sell shares you had prior to the split. This happens sometimes because conversion from old shares to the "new shares" under the CUSIP number hasn't been completed. I am not sure if it happens with all brokers or with every RS.
SGT Pepper is WRONG...the stock will have the letter D attached to it for 20 days from the date of designation to signify a reverse split is taking place. It does not stop trading.