Why is the fifteenth day of the first month also called Chinese New Year?

In most places in the history of Greater China, whether official or unofficial, the day from the end of the year (New Year's Eve) to the beginning of the year (New Year's Day) is regarded as the official "New Year's Day", commonly known as the New Year's Day. The Spring Festival is a cultural festival for China people about the annual time and the overall high priest, and it is also a festival for family reunion (family culture) and visiting relatives (family New Year greeting culture). The 45th day of the first month is obviously not the end of the year, nor can it be the Lunar New Year-China people can't be so stupid. On the contrary, the 23rd of the twelfth lunar month is generally called New Year's Day-in fact, it is to prepare for the New Year's Day on 16 or even 18, such as cleaning, preparing new year's goods, preparing sacrifices, sending the kitchen god to heaven and so on. In ancient times, it was just sixteen days from the day before New Year's Eve to the end of the fifteenth day of the first month, and the day before New Year's Eve was already full of ancient festivals. In addition, the 16th day of the first month is the Christmas day of the ancestors of Yi-ology, mankind and unification of Taifuxi, so it can also be said to be eighteen days. Because China people have always formed the custom of visiting the New Year, relatives have to go away for a month. It is precisely because the Chinese New Year Festival is the grandest and longest, so it is called Chinese New Year. Chinese New Year in a broad sense includes the Lantern Festival from the 14th to 16th of the first month (in some places, it is from the 13th to 15th of the first month), which developed into Shangyuan Festival after Taoism was founded. In some places, the Lantern Festival (Lantern Festival or Shangyuan Festival) is called New Year's Day, mainly because the New Year holiday in China does not end until the fifteenth day of the first month, and because the official holiday ends after the fifteenth day of the first month, it means that the "New Year's Day" which lasts for nearly sixteen days ends on the fifteenth day of the first month. The multi-material culture of ancient people was misinformed to the New Year with the passage of time. Of course, there may be a saying that the 15th day of the first month supplements the Spring Festival after Li Shimin's victory in the Anti-Japanese War, or something else. But the regular New Year's Eve and New Year's Day are still two days, and the generalized New Year's Eve is 16 days from the end of the year to the Lantern Festival on the fifteenth day of the first month.