Not actually true for Japan, Cambodia and Thailand.

In Japan, they celebrate New Year's Day (元日, Ganjitsu) on January 1st. The date for celebration of the new year was changed soon after the Meiji Restoration and the traditional festivities that were practiced on the 1st day of the Japanese lunarsolar calendar were transferred to January 1st. (I once mentioned this in conversation with a very tradition-minded Japanese friend; she didn't know about the change of date, was shocked and initially in disbelief until she checked it for herself and was shaken to realise that there was a discontinuity in the date used for the traditional celebrations).

In Thailand New Year's Day (Songkran) is always on April 13 and in Cambodia it is on April 13 or 14. These two countries have been influenced by the Hindu Calendar and April 13 marks the day when the sun (approximately) enters the Hindu equivalent of Aries, according to the sidereal zodiac system.