I have another take to add to Chad's and Greg's. When our clients wish to add threadfins, we want those fish to spawn as soon as possible. Hatcheries normally sell shad two different ways...full grown adults or young of the year. If the hatchery's shad made it through the winter, we can buy adults in spring. If not, it is late summer or even fall before young of the year are available. That's the hatchery side...with another twist. Some guys who sell shad actually have a permit to harvest them from public reservoirs when shad are "running the banks." These guys can catch lots of shad, haul them directly or stockpile them into smaller ponds to distribute over the next few weeks. Part of the reason there is a finite time to deliver shad is that there is usually a finite number of shad.
Now, for the consumer's side. Timing is really important. Remember that threadfin shad die when water temperatures drop into the lower 40's. As I recall, their lower lethal is 42. In north Texas and southern Oklahoma, that means threadfins will survive 4 of 5 winters. North of there, the odds drop, and drop fast.
So, pond owners must pay attention to the reasoning for stocking, their habitat and food chain for shad. Threadfins are primarily filter feeders, gleaning plankton from the water for nutrition. So, water should be fertile before stocking. Next, pond owners want shad to spawn, and spawn fast. So, we try to get them stocked as soon as the water is warm enough, and finish before shad have spawned out. That's the reason we push to finish by the middle of May around here.

Teach a man to grow fish...
He can teach to catch fish...