The Ex Dividend date is the date that you must own shares on to receive your percentage of the spinoff shares. It works the same way as when a stock pays dividends. There is a payout date, which is when the transaction occurs, and then the Ex Dividend date, which is the date you must be an owner of the stock.

In my experiences it is best to own before the Ex-Div date because stocks could be more volatile as people try to get in right before, or get out right after the date.