Rittou is one of the twenty four solar terms which divides a year into twenty four periods. Rittou refers to November 7th or 8th.  Using the moon as a reference autumn is still in full swing, however, people in ancient times regarded that winter began on Rittou.
The twenty four solar terms has been used since ancient times as a reference standard for describing the seasons since Japan adopted the lunar calendar.
Starting from Risshun (the beginning of Spring), until Daikan (Severe Cold), each of the four seasons are further divided into six solar terms.
Rittou indicates the arrival of winter as well as that winter has already begun.
The twenty four solar terms was well suited to Japan where the seasons have rather distinct beginnings and endings, but perhaps more to the point, people in old days lived a more agrarian life and could feel the subtle changing of the seasons. They must have sensed and respected the gentle flow of time that harmonizes them with the passing seasons.    
Slow down your life a little and you will appreciate the subtle changing of seasons through twenty-four solar terms.
After Rittou, arrives Shousetsu (Light Snow). Soon the snow starts falling.