Whenever your yard is fully prepped is the best time to spray. As stated above, the better prepped your yard is, the better the hydroseed is going to grow. That being said, each season has advantages and disadvantages to spraying. Spring and fall don't require as much watering because of the cooler air temperatures, but the grass tends to grow a little slower due to the cooler temps. In the summer the grass grows quickly, but requires a lot of watering to keep the mat moist.
When deciding when to spray, you'll also want to take into account when it will be easiest to keep animals, children, and visitors off your newly sprayed lawn. We suggest keeping the mulch mat undisturbed for at least 30-60 days to allow the seeds to establish a strong root base.