Free Dining has been offered, in the past, in April. Remember, there is no guarantee that this will be offered again. And as I recall, the available period gets smaller each year. This time I think it was only a 4 week period. I am convinced that the growing number of people who book rooms for the anticipated free dining time with the expectation of switching to the free dining, have had a detrimental effect on the offering. After all, free dining is an attempt to fill up Disney Resort rooms. If there are no unbooked rooms then there is no need to offer it.
That said, keep your eyes open next April and be ready to travel at the very end of August or in Sept.