There are 2-3 tea harvests in Japan each year. The first harvest is in late April or early May. There are 1 or 2 more harvest during the summer.
Japanese Green Tea Online only ships first-harvest teas
There is a tremendous difference in quality between teas from the first harvest and all the rest.
First-harvest teas not only taste better they also contain more polyphenols and vitamins than second and third harvest teas.