This is from the Marriott T&Cs for the MegaBonus promotion:

Do stays made prior to registration count toward the promotion?

Yes, as long as you register during the promotion, and a check-in or check-out date falls within the promotion date range. The only exception is if you check out on the day the promotion begins, 2/15/06, then you will not be eligible for bonus points for that stay.

From this, I would say that you will receive credit for your stay on May 15th as your checkin date is within the date range.