The traditional East Asian calendars divide a year into 24 solar terms (节气). Grain rain (谷雨Gǔyù) is the 6th solar term. It begins when the Sun reaches the celestial longitude of 30° and ends when it reaches the longitude of 45°. It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 30°. In the Gregorian calendar, it usually begins around April 20 and ends around May 5.

"The arrival of Grain Rain signifies the departing spring." This means that Grain Rain is the last solar terms in spring. Each year, between April 19th and April 21st, when the sun reaches 30 degrees of ecliptic longitude, the newly-transplanted seedlings and newly-sowed crops are most in need of the soaking of rain water and the trees have been full of peaches, pears and apricots on their branches. At the same time, duckweed begins to burgeon and grow, exuding a flourishing and thriving sight.

In ancient China, there were three indications that Grain Rain season is coming: "The first indication is the burgeoning duckweed; the second one is the chirping cuckoos preening their feathers; and the third one is the hoopoe on the white mulberry trees.' That is to say, on the day of the Grain Rain, some duckweed appears in the ponds, and the cuckoos begin to chirp on the branches five days later, and the hoopoes will fly on the mulberry trees five days after that.

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