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As the 16th of China's 24 traditional solar terms, the Autumnal Equinox represents a pivotal point in the natural cycle and embodies profound cultural significance. The ancient text Collected Explanations of the Seventy-Two Pentads offers a clear definition: "The Autumnal Equinox begins in the mid-eighth lunar month." It further explains, "The term 'Fen' (分) means 'to divide equally,' marking the midpoint of autumn's ninety days." Here, "Fen" signifies an equal division, indicating not only that autumn has reached its halfway point but also that day and night are approximately equal in length. Around this time, the sharp contrast between hot and cold temperatures gradually softens.

From an astronomical and calendrical perspective, the Autumnal Equinox typically occurs around September 23rd each year. At this time, the handle of the Big Dipper points to the "You" position (aligned with west in traditional Chinese orientation), and the sun reaches the celestial longitude of 180°, known as the "Autumnal Equinox Point." After this moment, the sun's direct rays shift southward from the equator towards the Southern Hemisphere. Consequently, the Northern Hemisphere experiences gradually shorter days and longer nights. Temperatures begin to drop steadily, the air becomes crisper and drier, and the characteristic season of "crisp autumn skies" officially arrives.

Culturally, the Autumnal Equinox was historically an important "Moon Worship Festival" in ancient China. The ancients viewed the moon as a symbol of fullness and good fortune, making offerings and holding ceremonies on this day to pray for bountiful harvests and family safety. The well-known Mid-Autumn Festival is deeply rooted in this traditional custom of moon worship. Historical research suggests the original Moon Worship Festival was indeed held on the Autumnal Equinox. However, because the moon is not always full on this specific date—while the ritual strongly symbolizes the "full moon"—the celebration was later shifted to the 15th day of the eighth lunar month. This date typically guarantees a bright, full moon, eventually evolving into the millennia-old Mid-Autumn Festival we know today.