The ex-dividend date is a day on which stock goes ex-dividend.
The ex-dividend date for stocks is usually set one business day before the record date.
On September 9, 2017, Company XYZ declares a dividend payable on October 4, 2017, to its shareholders. XYZ also announces that shareholders of record on the company's books on or before September 19, 2017, are entitled to the dividend. The stock would then go ex-dividend one business day before the record date