Relationship to the record date
Because settlement of a stock trade takes a certain number of business days from the trade date, purchasing shares on the record date itself would not allow enough time to be recorded in the shareholder register. As a result, the ex-dividend date falls two business days before the record date, meaning shares must be purchased before that date (by the last trading day with rights attached).
What to keep in mind
On the ex-dividend date, selling by investors who had held shares for the dividend or benefit is sometimes cited as a reason share prices tend to soften. However, this is also influenced by overall market conditions and the situation of the individual stock, so it does not guarantee any uniform price movement.