The ethnic groups in North China gradually invented a kind of phenology calendar that shows year and season by natural phenomena in the process of fishing and hunting to guide production and life.
北方民族在长期的渔猎生产中逐渐形成了以物候确定年岁、节的自然物候历，用以指导人们的生产与生活。
2
Because of this, every 4 years an extra day is added to the calendar so we don't fall behind in the natural cycle of things.