Chinese plum/Japanese apricot trees (Prunus mume) are native to China and came to Japan about 1,500 years ago. Since then, Chinese plum trees have been cherished (e.g., blossoms viewed and fruits eaten) by Japanese people. Today, a symbol of spring in Japan is cherry blossoms but during the Nara period (710 to 794), it was definitely Prunus mume blossoms as supported by the fact that Mannyōshū (i.e., the oldest extant collection of Japanese poetry compiled during the Nara period) includes more poems about plum blossoms than those about cherry blossoms.
Chinese plum trees generally start to bloom in January but some do so in late December.