This is what I found on Wikipedia about CNY: People typically wear new clothes from head to toe to symbolize a new beginning. Wearing new clothes also symbolizes having more than enough things to use and wear in the new year. Wearing red is preferable because it will scare away evil spirits and bad fortune.
For a few years in the row, I haven't put on new clothes or cut my hair on the first day of CNY. I should start doing it next year! Besides new hair cut and clothes, it is also beneficial to get a new wallet to ensure not only continuity of good fortune, but also to attract new money

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As for the consequences of not having a new hair cut and wearing old clothes, hopefully we will get some answers.