Pardon the slang, please, maybe it's the heat...
July and August are festival season. It's blazing hot, the drums are pounding, and you have to get up and dance. What do you wear to dance in a Japanese summer festival? A yukata, of course.
The yu in yukata is the symbol for hot water. It doesn't mean that dancing at a festival while wearing a yukata can land you in hot water. It means that the yukata was originally a kind of bathrobe, something to put on when you emerged from a hot springs bath.
Yukata are still used for that purpose--very light cotton, almost always in a blue and white pattern. If you stay at a hot springs resort, they are pretty much the uniform. You'll see guests wearing them to dinner in the main dining room.
However, there is another kind of yukata. To see one is to want one. While dark blue--the color of a lake when the sun is going down--is still the preferred background color, it isn't the only color. The dyed-in patterns range from subtle to neon, and the whole thing is tied up in an obi that spreads beneath your shoulder blades like the wings of a butterfly.
At least the young women's obi is. Men and older women tie a narrower obi around their waists like an ordinary sash.
As for the men, they have two choices. One is the kimono-like yukata. The other is a knee length pajama-like outfit called jinbei. The tops and bottoms match, like a suit, and if you ask them, the men will say they would like to ditch their modern suits and wear jinbei to work instead.
In the old days when farmers and craftsmen ruled the workplace, I suppose they did.