The promotion runs from May 15 until September 2. Since you selected and were given by the platinum line the certificate offer, depending on which one you were granted (Category 4 or Category 5 certificates), here's the layout:
For each two stays during the promo period you will either get one Category 4 certificate up to earning three certificates (six stays) or you will get one Category 5 certificate up to earning two certificates (4 stays). These certificates have a one year expiration date.
Track your stays and once you have two stays earning a certificate, you can call the plat line and often (not always, because that's just the way it is in the Marriott world of bonus esoterics) they will credit you a certificate as soon as you earn it, allowing you to use it promptly if you so desire (if not, you may just want to wait until the end of the promo, thus lengthening the life of your certificate/s).
Remember the certificates are earned with stays not nights, and as a platinum you know the difference.