Many westerners question why the Chinese New Year's Day is not a fixed date in the calendar. It's sometimes in January, sometimes in February, how is it determined? Some say that the Chinese New Year's Day is adjusted according to the leap month so that it falls around the Lichun day (first day of spring). That's why the Chinese New Year is also called the Spring Festival.
 
This is a complete misunderstanding. The traditional "Spring Festival" is on the Lichun day and it is a completely different festival than the Chinese New Year. Generally speaking, the Chinese New Year's Day is the second new moon day after the winter solstice (December 21 or 22, the shortest day of the year), except in some special cases. The new moon day is the day in the lunar phase when the moon cannot be seen. The traditional Chinese calendar uses the new moon day as the first day of a month. The opposite of new moon is the full moon and the full moon day is normally the 15th or 16th of the month.
 
Why is the second new moon day after the winter solstice generally the Chinese New Year's Day? We have to start with the introduction of the Chinese calendar.
 
Many people call the Chinese calendar the lunar calendar. This is a mistake as well. The true lunar calendar only uses the lunar phase in its calculation which is also called the tai-yin calendar. The period of a lunar phase is about 29.5 days. It always has 12 months in a year and about 354 days in a year. Therefore, after 33 years, there will be a one-year difference from the current calendar. Corresponding to the tai-yin calendar is the tai-yang calendar, also called the solar calendar. Its calculation is based on the cycle of the Earth rotating the sun. One cycle lasts about 365.25 days and is called a solar year. The days in a year are determined this way, either 365 days or 366 days. The current standard calendar used in the world is a kind of tai-yang calendar as it is completely based on the cycle of the earth rotating the sun. The months in a year has nothing to do with the lunar phase.
 
Both the pure tai-yang and tai-yin calendars have their advantages and disadvantages, since both the sun and moon are highly important astronomical objects for humans. On one hand, the cycle of the earth rotating the sun correlates with the four seasons and it has a great impact on humanity and the growth of organisms on the earth. On the other hand, modern science has discovered that the lunar phase impacts human physiology and social behavior. Although modern science does not have an explanation of why and how, scientists have confirmed from their observations that women's menstrual cycles, disease incidence rates, crime rates, traffic accident rates and even people's diet changes all relate to lunar phases.
 
The Chinese calendar is not a pure lunar calendar or a pure solar calendar, but a combination of both. In ancient China, the tai-yin (moon) and the tai-yang (sun) are the two most observable objects in the sky, thus neither of them could be ignored. The ancient Chinese were particular about the unification of heaven and humans. From a different perspective than modern science, they paid extra attention to the movement of planets and their intertwining relations with human society. The Tao culture has influenced China for more than five thousand years, and one of Tao's important concepts is the balance of yin and yang, so the Chinese calendar uses the lunar phase to define the month, and there are 12 months in a year. After a fixed period of time, a leap month is added so that the average days in a year is close to a solar year. Moreover, the sun's movement cycle is divided into 24 solar terms with each term being 15 degrees to reflect the four seasonal changes. The calculation of a leap month is also according to the solar terms. Therefore, the Chinese calendar contains both yin and yang calendars. Today, almost all the Chinese in the world, plus Korea and Vietnam, use the Chinese traditional calendar to calculate the dates for the New Year, Mid-Autumn Festival, Dragon Boat Festival and Qingming Festival. Other countries have also developed a calendar combining yin and yang (lunar and solar) over the course of history, but none have a calendar that exceeded four solar terms. Only the Chinese calendar uniquely and accurately developed the 24 solar terms. In the Yuan Dynasty (1271 – 1368 AD), Guo Shoujing wrote the book Teaching Time and Calendar, and it calculated the solar year as 365.2425 days. This is the same as the Gregorian Calendar, which is widely used in the world today, but was developed three hundred years earlier.
 
So which month does a year begin with? In ancient China, ten Heavenly Columns and twelve Earthly Rows were combined to count year, month, day and time period. These have a close relationship with the five elements: metal, wood, water, fire, earth. The twelve Earthly Rows correspond to the twelve months. The Chinese calendar uses the month of the winter solstice as the "Zi" month. In the Earthly branches, Zi (first Earthly Row) is water, which indicates that yin is at its peak and starts to decline, and yang strengthens. It is the same concept as calling midnight the Zi time period (2 hours per period) in ancient China.
 
Although the months correspond to the Earthly Rows, the New Year's Day still had to be decided. Five thousand years ago, the Yellow Emperor (2698 – 2599 BC) built the first calendar. Later, various calendars were set up in different dynasties and the date of the New Year varied. From the time of Emperor Wu of the Han Dynasty (156 – 87 BC) until now, the tradition of setting the Yin (third Earthly Row) month as the first month of a year was kept, so the first month of the year is the second month after the month of the winter solstice (Zi month). Therefore, the Chinese New Year's Day is the second new moon day after the winter solstice. There are also exceptions, such as when the leap month happens in the 11th month of the year. However, this situation only happens about every 200 years.
 
There is no reason to connect the Spring Festival with the Chinese New Year. During the late Qing dynasty time when Yuan Shikai (1859-1916 AD) was in power, the Chinese New Year's Day was renamed as the Spring Festival. But the Chinese people didn't accept it, and they still celebrated the Chinese New Year in the traditional way. After 1949, the Chinese Communist Party forcefully implemented this among Chinese people and the Spring Festival replaced the Chinese New Year, thus destroying one of China's traditions.

传统历法中的新年是怎么确定的
 
很多西方人有疑惑，为什么每年的中国新年在公历中的日子不定。中国新年一会儿在公历一月份、一会儿在二月份，到底是怎么推算的呢？有人解释说，是通过闰月的调整尽可能使得大年初一日在立春前后，因此称为“春节”。
 
其实这完全是误解。传统上“春节”指的就是节气中的立春，跟新年根本不是一回事。要简单讲起来，除了很少见的特殊情况外，一般大年初一日可以从冬至日（大约是每年公历的十二月二十一、二十二日，这一天白天最短）之后的第二个朔日来推算。这里说的朔日是指月相盈亏变化周期（朔望月）中，看不到月亮的这一天，也称新月，传统历法把这一天定为每月初一日。与此相对的是月圆之日──望日，一般是每月的十五、十六。
 
为什么一般来说冬至日之后的第二个朔日为大年初一呢？这要先简单说一下中国的传统历法。
 
很多人把中国传统历法称作阴历，这也是一个错误的叫法。真正的阴历是单纯以月相的盈亏周期（大约二十九点五天）为基础的，也称太阴历，每年固定十二个月，大约三百五十四天，所以积三十三年与公历相差一年。与太阴历相对应的太阳历（简称阳历），是单纯以地球绕太阳的周期运转制定的历法。以地球为基点观察这个周期运转，则表现为太阳在黄道上的运行（太阳的视运行），这个周期即一个回归年（大约三百六十五点二五天），以此确定每年的天数，或三百六十五天，或三百六十六天。现在世界通用的公历就是一种太阳历，它完全以地球绕太阳公转的周期为基础，其月份的划分与月相的盈亏没有关系，只是个名称而已。
 
单纯的太阳历和太阴历各有其利弊，因为太阳和太阴（月亮）都是对人类至关重要的天体。一方面和太阳视运行周期相关的四季变化对人类和万物生长有很大影响，另一方面现代科学逐步揭示出，月相变化和人类的生理、社会行为也息息相关：虽然现代科学还无法解释，但科学家发现统计数据表明，女性的生理周期、很多疾病的发病率、犯罪率、交通事故率，乃至饮食量的变化都与月相盈亏有明显相关。
 
中国传统历法既不是单纯的阴历，也不是单纯的阳历，而是一种阴阳合历。因为对于古人来说，太阴和太阳作为天上最显眼的两个天体，都不容忽视。中国古人讲究天人合一，从不同于现代科学的另一个角度特别强调了天体运行和人类社会的紧密关系。而从贯穿五千年中华文明的道家文化来讲，阴阳平衡是一个极其重要的概念。因此传统历法以朔望月确定历月，每年十二个月，每隔一定时间要增加一个闰月，使历年的平均长度接近回归年，此外又根据太阳在黄道上的周期视运行，以十五度等分划分出二十四节气，对应四季变化。闰月的推算也要依据节气。所以传统历法中兼具阴历和阳历的成份。至今几乎全世界所有华人及朝鲜、韩国和越南等国家，仍使用中国传统历法来推算传统节日如新年、中秋节、端午节、清明节等节日。历史上虽然世界其它民族也有发展出阴阳合历的，但对于节气，最多只有四分的，二十四节气的准确划分为中国历法独创；而且早在元代郭守敬编订的《授时历》就测定出一个回归年为三百六十五点二四二五日，和现在世界上通用的《格里高利历》（即公历）的周期一样，但比后者早了三百多年。
 
那么一年从哪一个月开始呢？古时以干支纪年、月、日、时，其中“子丑寅卯辰巳午未申酉戌亥”十二个地支恰好和十二个月对应。历法中把冬至日所在的月份定为子月。这是因为天干地支和阴阳五行相关联，地支中的子属水，对应阴气盛极而开始渐衰、阳气初生的状态，而冬至这一天恰好是黑夜达到最长，从此以后开始黑夜逐渐变短，白天逐渐变长，所以和地支中的子对应。这和半夜对应子时是一个道理。
 
月份和地支的对应完成后，仍然存在确定新年起始的问题。自从五千年前轩辕黄帝创制第一部历法后，中国历代曾颁布过许多历法，也曾有过不同的新年起始规定。如夏历以寅月为正月（建寅），殷历以丑月为正月，汉武帝至今大多数时间都沿用了夏历建寅月的传统。这样正月（寅月）就是冬至所在月份（子月）之后的第二个月，所以新年正月初一就是冬至之后第二个朔日。当然也有例外的情况，如出现闰十一月的时候，但这种情况大约要隔二百年才会出现一次。
 
由此可见，把“春节”或立春和新年扯在一起是没有根据的。把新年的正月初一改称“春节”，始于袁世凯主政期间，但老百姓并不买帐，照旧“过新年”。“春节”一词真正在民间取代“过年”还是在一九四九年之后中共以政治原因强制实行的，从而破坏了中国古老的传统之一。