In the picture she is wearing Japanese Yukata, which is a Japanese traditional summer Kimono made of cotton or synthetic fabric. Like other forms of Traditional Japanese clothing, Yukata are worn by both men and women.
I assume Yukata and Kimono were commonly worn daily clothing for Japanese people until before WWII. Because My grandmother (100 years old) has so many kimono and Yukata in her closet, and I see pictures of my grandparents wearing Kimono when they were young. The Japanese still have a trend to wear it after they took a bath at night as a pajama. Yukata literally means Bath-clothes, however, it is commonly worn during the hot summer months for occasional events such as Hanabi (fireworks) displays and Bon-odori festivals.