A company will declare a "book closure" to signify a date. Records are still kept during this period but the beginning date, or book closure date, is important for determining who will be affected by some future change, for example it helps determine who gets the dividend. A stock which pays a dividend often increases in price by the amount of the dividend as the book closure date approaches. Due to the logistics of processing the large number of payments, the dividend may not actually be paid until a few days later. After the book closure date, the price of the stock usually drops by the amount of the dividend, since buyers after this date are no longer entitled to the dividend.
Investment dictionary. Academic. 2012.