Yukata is...

Yukata is a kimono for summer festival such as fireworks, temple festivals and bon dancing.
Most women wear yukata for those events.
In order to create cheerful mood in a hot spa, some villages in Japan welcome visitors wear yukata during their stay.
Wearing yukata is getting popular during summer festival and hot spa areas
due to its colorfulness and nice sound by Japanese clogs(Geta).
In traditional Japanese style hotel 'Ryokan', service staffs have started to wear yukata as a uniform for summer or even whole season.
Yukata is used to be a pajama at a ryokan (hotel) and western style hotel.
It is also used as an uniform for Japanese practices.

Yukata meant that wearing a cloth after taking a bath, originated from a cloth for entering a spa at a temple.
At that time, most of spas used to sauna style bathes so that people wore yukata there. It is different from current public bathhouses in Japan.
Yukata now means Chyuugata (Cotton yukata), be worn after taking a bath for cooling.