The saying that "getting a haircut during the Spring Festival will kill one's uncle" is actually a misinterpretation based on a historical event and a homophone. It originated from the mandatory haircutting policy imposed by the Qing Dynasty on the Han people, during which time many Han people expressed their nostalgia for the previous dynasty by saying "thinking of the old times" (思旧), which was later mispronounced and spread as "killing the uncle" (死舅) due to a homophonic mistake. Therefore, this is a folk taboo without scientific basis.

I shall share the edition I heard.

The story started in Qing dynasty.

In 1644, Ming dynasty was replaced by Qing. The emperor of Qing made all people cut their hair and change their original hairstyle into a new hairstyle which is known as the long braids. People had nothing to do but  to obey the law,otherwise they might lose their heads. People at that time were angry but they had got no choice.

Therefore, some people began to recall the past, when Ming was still in charge of the country and their original lifestyle was kept. During the Chinese new year, they began to cut their long braids, in order to commemorate the past and express their anger.In Chinese," commemorate the past (思旧)" and "make uncles die" almost share the same pronunciation. So the saying of "cut hair during the new year to commemorate the past" was spoken incorrectly by some people and the saying changed into "cut hair during the new year to make uncles die". And this saying remains.

This is why some Chinese people say that cutting your hair in Chinese new year will bring your uncles bad luck.

The saying "Cutting one's hair in the first lunar month leads to the death of one's uncle" has its origin in the Qing Dynasty. The main viewpoints regarding its origin are as follows:
 
Resistance to the Haircutting Order: After the Qing army entered the pass, the Shunzhi Emperor issued the "Haircutting Order" in 1644, compelling all officials and common people to shave their heads and wear queues, replacing the Han people's traditional hairstyles with Manchu ones. This policy was strongly opposed by the Han people, who believed that cutting their hair was an insult to Han culture and a betrayal of their ancestors' traditions. Therefore, many Han people passively resisted by not cutting their hair in the first lunar month, expressing their nostalgia for the Ming Dynasty, which was called "Sijiu" (longing for the past).

Evolution from Homophony: As time passed, due to the similar pronunciation of "Sijiu" and "Sijiujiu" (uncle dies), and the special respect for uncles in ancient society, this custom gradually evolved into the saying "Cutting one's hair in the first lunar month leads to the death of one's uncle". This statement, though baseless, was easy to remember and spread widely.

Social and Cultural Factors: In traditional ethical concepts, people always hold that "Mothers' brothers are of great importance". This saying may have been emphasized and spread because of people's emphasis on family ethics and respect for uncles.

In fact, this is a misunderstanding. During the Qing Dynasty, people were required to cut their hair to mark the change of the dynasty. However, many people refused to cut their hair in the first month of the year as a way of rejecting the new dynasty and cherishing the old one.

The phrase 'cherishing the old dynasty' sounds similar to 'uncle died' in Chinese. Over time, this phonetic similarity led to the belief that cutting hair in the first month of the new year symbolizes the death of an uncle.

 #SpringFestival Knowledge Contest

The saying "cutting hair during the Chinese New Year brings bad luck to your uncle" is rooted in Chinese superstition and wordplay. In Chinese, the word for "hair" (发, fà) sounds similar to the word for "prosperity" (发, fā). Getting a haircut during the New Year is believed to "cut off" prosperity. Additionally, the phrase "bad luck to your uncle" (妨舅, fáng jiù) is a homophone for "hair cutting" (理发, lǐ fà) in some dialects. This playful association has led to the belief that getting a haircut during this time could bring misfortune, particularly to one's maternal uncle.

It's from the Qing dynasty. #SpringFestival Knowledge Contest.

I have no idea 

Because in Chinese "cuttting hair" sounds like "dead uncle".

No, during the Spring Festival holiday, which lasts the entire lunar January in my mind. However, I think it's just a joke to add to the Spring Festival atmosphere and enjoy quality time with family.

This is superstition. It's not true. Please don't believe it！#SpringFestival Knowledge Contest