What is the Spring Equinox, which was also called "Midday" or "Day and Night Division" in ancient times? It occurs on March 20th or 21st each year when the sun reaches the celestial longitude of 0°. According to *Collected Interpretations of the Seventy-Two候of the Monthly Ordinances*: "In the second month, division means half. This is the halfway point of the ninety days, hence it is called 'division'." Additionally, in *The Luxuriant Dew of the Spring and Autumn Annals: Yin-Yang Exits and Enters, Upper and Lower Chapter*, it states: "At the Spring Equinox, yin and yang are equally divided, thus day and night are equal and cold and heat are balanced." Therefore, the significance of the Spring Equinox has two aspects: first, it refers to the day being evenly divided into 12 hours of daytime and 12 hours of nighttime; second, in ancient times, spring was considered to last from Start of Spring to Start of Summer, and the Spring Equinox falls right in the middle of these three months of spring, dividing the season equally.
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