Ex-Date is the date used as basis for qualification to stock rights offer and/or dividends. To qualify, the shares must be bought and held at least 1 business day before the ex-date.
Example:
Stock "A" has declared a stock dividend with Ex-Date September 23, 2016. This means that all shareholders of the stock at the end of trading on September 22, 2016 will receive a dividend based on the number of shares held. Shares acquired on September 23 and onwards are not qualified for the dividend. Shares sold on or after the Ex-Date still qualify for the dividend as long as the shares were bought before the Ex-date.