You must own before the ex-dividend date; therefore, your must purchase on May 2
Ex dividend date is Fri, May 3, 2013, 3:00AM EDT
And, as has been pointed out before, you may have to buy 3 days before the ex-dividend date so that the settlement is completed by May 3rd.
I have not traded too close to ex-dividend dates. Other knowledgeable people may be able to confirm this.