I've just checked my account and have been able to register for
"5,000 Marriott Rewards bonus points after every other paid stay – starting with your second stay. You can earn a total of 15,000 points." June 1st - Aug 31st
Not overly exciting.
This is the third promotion on the go; I've also got the Double Nights promotion (May 11 - June 26) and the Marriott Rewards Enrollment Offer (May 1 - July 31)
I'm not complaining, but it's all a bit baffling.