The 88th night counted from the first day of spring, falling on 1 or 2 May depending on the year, traditionally the marker for the first harvest. A well-known Japanese song opens with it, and folk belief holds that tea picked on this day brings long life.
In more detail
The 88th night from the first day of spring, falling on 1 or 2 May. It marks roughly the point where frost stops being a threat, and was used as a marker for farm work generally, not only tea — though a well-known school song about tea picking has fixed it in the popular mind. Actual harvest dates vary widely by region; Kagoshima starts in early April.