This year it has been officially announced to start from the 1st of June and this usually continues till the end of September.
No, the Japanese people do not shed their skins but rather go for a change of clothes. The literal translation means a change of wardrobe.
This started with the Cool Biz campaign in the summer of 2005 when the then Japanese Ministry of Environment minister, Ms. Koike Yuriko proposed the idea to help reduce electric consumption in summer resulting in reduction of CO2 emission. Apparently, British and South Korean firms carried out the idea in 2006.
People wear short sleeved shirts without neckties or jackets to work and the air conditioning is set at 28 degrees. This year Aloha or Hawaiian shirts are the boom as recommended by several organizations.
Japanese society has many kinds of pressures and conformity is an absolute must. A few years ago, the dress code at the work place was almost as strictly oppressive as probably in the military. All had to be in suits. But it is very difficult to work wearing a suit in the hot evervating summer. It also requires that the air conditioning be turned on and temperatures kept low. This requires huge amounts of electricity. So it was proposed that people wear ligher clothing and the bare minimum of air conditing be used. I feel this has done a world of good to Japan where formalities can be very stifling. Japanese people try and bear all that stoically, but in the age of globalization it would not do much harm to relax rules a bit and bring in a bit of color (at least one season of it). After all, I have always felt that Japanese people are trying to be more Western than Westerners when it comes to wearing suits.