Curious too - I called and asked about checking in on march 7th, and the person I spoke too thought we'd only get the discount that one day (as its the last day of the promo) not the whole week -- of this was wrong I'd love to know!
I can tell you that it didn't work this way for us for Spring 2012.
This past Spring they had free dining, and it ended on March 9th. We checked in on March 7th and were able to get the free dining discount for our whole trip. The CM told me that if your check in date was during the elligible time frame, you can apply it to your whole trip - we just had to check in no later than 3/9th.
So unless the rules/procedures have changed you should be fine.