Why is there a day of yin and yang?

Why are you talented? Because the four pillars and eight characters all come from the combination of heavenly stems and earthly branches, there will be annual branches, monthly branches, daily branches and time branches. On the other hand, it is composed of heavenly stems and earthly branches with the same composition. Whether in numerology, date selection or other Yi studies, Yin and Yang have always existed and are inseparable. Where there is yang, there is yin. Ten days are dry, five yin and five yang. There are twelve branches, including six yin and six yang. In Yi-ology, odd numbers are Yang, even numbers are Yin, and the fixed collocation is Yin with Yin and Yang with Yang. For example, Jiazi Day and Yihai Day are the same in nature, but they are yin and yang. A is the first in heaven, and it is strange, and the son is also the first in the earth, and it is strange, and the strange is Yang, and Yang matches Yang. B is the number of days, even number, even number is yin, and yin matches yin. Therefore, although A and B belong to wood in the five elements, Zi and Hai belong to water in the five elements, they belong to Yin and Yang respectively.