Koromogae is the seasonal change of clothes. It came from China in the Heian period as a court custom of changing clothes twice a year. Today the custom is to change from winter to summer clothes around June 1 and from summer to winter clothes around October 1. After Western uniforms came in during the Meiji period, officials and soldiers changed twice a year, and the custom spread to schools and homes.
In the summer of 2005, the Ministry of the Environment began to promote Cool Biz, light office clothing with air conditioning set to 28 degrees Celsius. At first it ran from June 1 to September 30. Later the period changed.