In April the trees behind the wall do the only showy thing they do all year, and then think better of it. The blossom lasts a week, sometimes less, and is gone before most of the people it is kept for have seen it. This is held to be part of the point.
Pomona was the one the old writers gave to fruit trees and to the tending of them: not the harvest, which anyone can enjoy, but the patient middle of the year that makes a harvest possible. It is an unglamorous office. Most of the work is pruning, waiting and keeping animals out.
We mark April rather than August because the beginning of a thing is more honest than its reward. By the time there is fruit to speak of, the season has already decided what it will give. In April it is still only a promise, which is the part worth writing down.