Certified GKH Gardening Expert
Ideally, you will fertilize between each season. You can get away with once per year, though. It isn't going to be picky about when, but won't use it much during dormancy.
If you are going to fertilize before a rain, then make sure that your feed is granular. This will ensure that it makes its way into the ground, slowly, with each rain.
Here is an article for more information on Zoysia: