Yukata

In the old Japan, during the Heian period (794–1185), the way of taking a bath was quite different from today. They were taking steam baths and the nobility used to wear a robe when they were taking the bath, to avoid contact with the hot steam. That robe was made of cotton and it was called yukatabira (from yu which means hot water and katabira which is a robe worn under the clothes).

Later, during the Edo period, the bath style changed and people started wearing the robe after the bath (and it was called yukata, short from yukatabira)
From here to the street usage it was only another step. Today, yukata is a casual summer clothing, worn by men and women during festivities or dates...

Traditionally, yukata were colored with indigo patterns (that's why many Japanese hotels supply guests with indigo yukata). But today yukata are made in an astonishing variety of colors and patterns - here are a few beautiful models photographed in Shinjuku: