there is an old custom of changing the kinds of clothing
one wears to the season
. The two major times for this are June 1st when lighter and cooler clothing is worn, and October 1st when heavier, warmer clothing is donned.
It does not matter what the temperature actually is or what the weather is like, it is the day to "change clothing". Of course, this sounds silly. But koromagae is a formal acknowledement of the change of seasons, of the impermanence and tenderness of our lives. But like many such customs, rather than being practised as something to enliven and awaken a recognition, they become part of rote behaivours, actions, and attitudes.
Even school uniforms tend to be dark navy blue or black during the winter time and a lighter colour in the summer.
Of course, kids these days don't observe koromagae. They start wearing shorts ("shotto shatzu") and short skirts as soon as they can.