EDO TOKYO Digital Museum Home Historical Visit, New Wisdom.

People in Edo period lived their lives according to the "lunisolar calendar". As this term suggests, this calendar indicates both the moon phases and the time of the solar year.
It takes 29.5 days to travel around the Earth. Thus, each month consists of either 29 days or 30 days. Some months consisting of 29 days are called ‘shōnotsuki’ (literally means "smaller month"), whereas other months consisting of 30 days are called ‘dainotsuki’ (literally means "bigger month"). Furthermore, it takes 365.25 days for the Earth to travel around the sun. While this gives an indication of the season, the repetition of the shōnotsuki and dainotsuki months will lead to a discrepancy between calendar and season. Thus, although most years should require only twelve months, once in two or three years, a thirteenth month is added.
However, even this method cannot prevent discrepancy emerging between season and calendar. In order to overcome this problem, the solar term (Nijūshisekki) was used. One year is divided into 24 solar terms, regardless of the movement of the moon. Of these 24 solar terms, the term, Risshun refers to the beginning of spring, Rikka to the beginning of summer, Risshū to the beginning of autumn, and Rittō to the beginning of winter.