If it's the promotion that I'm thinking of.....it's called Taste of Platinum. If you register for the program, you have 3 months + whatever is left of the month that you are in to stay 20 nights at a Marriott property. During the promotion phase, you will be ungraded to Platinum status, and upon completing the required number of nights, you will be given Platinum status for the remainder of the current calendar year, plus the following calendar year.
But definately call the Customer Service number and ask about the promotion. Good luck!
kingkamea is right. You can also do a Taste of Gold which would require less nights to meet the challenge. You can also try the Taste of Platinum or Gold from any lower level. For example, you might be a Silver Elite, but can go straight for the Taste of Platinum. Best to do one of these a little before you expect some stays that will give you several nights towards your challenge. You do get to enjoy the level you going to do the taste of, while you are trying to meet your challenge too. That's the nice part.