What is the Ex-Div Date?
For purposes of receiving a dividend from a share of stock or a mutual fund there is only one thing you need to know:You must buy the stock Before the Ex-Div date.
Why? Because the Ex-Div date is the day the stock begins trading 'EXcept the DIVidend'
On the ex-div date the stock virtually always opens lower by exactly the amount of the dividend.
This is because no-one who buys the stock that day can receive the dividend.
Ref: http://www.squidoo.com