The details of the summer Megabonus are as follows based on what I have heard:
Earning dates for this offer are from May 27, 2017 to September 4, 2017. If you enroll in this promotion, you will earn 2,000 bonus points for each paid stay, beginning with your second paid stay during the promotional period.
You may also earn an additional 2,000 bonus points by staying at different Marriott Rewards participating hotel brands. You will receive an additional 2,000 bonus points for each different brand that you stay in during the earning dates of the promotion, starting with the second brand.