Why do Japanese celebrate Western New Year and not Chinese New Year?
Japan had adopted the Gregorian calendar in 1873 as above comment said. It was one of the policies of the Meiji Restoration of 1867. The government determined to model the state after Western countries, instead of China. Since 1873, all the Japanese (including Shinto priest and Buddhist monk) have been celebrating New Year at 1st January, because they thought it’s rational. There are no relationships with WW2. A postscript: The Japanese have been celebrating New Year at 1st January since ancient times. People celebrated New Year at 1st January 1872 by lunisolar calendar. People celebrated New Year at 1st January 1873 by Gregorian calendar. What had changed was not the tradition but calender. Why must we celebrate other day?
