The scientists knew when the supernova would appear because they had already seen it.
The supernova nicknamed Refsdal was observed using a gravitational lens, and was located within a phenomena called an Einstein Cross caused by a galaxy within a super cluster.
"While studying the supernova, we realized that the galaxy in which it exploded is already known to be a galaxy that is being lensed by the cluster," explains Steve Rodney, co-author, from the University of South Carolina. "The supernova's host galaxy appears to us in at least three distinct images caused by the warping mass of the galaxy cluster." Link
Because of the uneven distribution of mass within the super cluster light from the host galaxy travels to us by multiple different paths of different lengths.
The astronomers were able to make precise predictions for the reappearance of the supernova in one of the other locations. They even predicted that light from the supernova had reached us in 1998, but we had missed it.
On 11 December Refsdal finally made its predicted, but nonetheless showstopping, reappearance.