Yukata?

When I tell people I make yukata, many people have no idea what I am talking about. You say kimono, and most non-Japanese people at least have an idea about what you're talking about, but yukata? What is that?

Yukata were actually originally robes worn at hot springs, much like dressing gowns here in the west. As time went on they became clothing in their own right, often worn at summer festivals. Think of them as the blue jeans of the kimono family; appropriate for play and work, but you can't wear them to a black-tie affair^^ They are actually seeing a return to popularity by younger generations, possibly because thy are easier to wear and care for, and with fewer 'rules' on proper patterns and colors, than formal kimono.
For a long time yukata were almost always made of cotton, with white, blue or bluish gray patterns. Now they are made in a variety of bright colors and patterns, 'though cotton is still by far the fabric of choice. Most men's yukata are still fairly plain, with geometric or water-themed prints, whereas women's yukata are often floral patterned. Children's yukata can end up having just about anything on them, from solid colored to toys, Mickey Mouse and Hello Kitty-chan.

There are actually several different patterns with which yukata are sewn.
First, women's yukata are open at the arm pit, sleeves and body both. This is to allow the wearer to adjust the fabric around and under the obi so that everything lies smooth. the fronts of the women's sleeves are usually curved at the bottom hem, and may be slightly longer than the men's.
Women's yukata come in two standard lengths; ankle and 'ohashori.' The second is considered more formal or proper. It is as long as the woman is tall; the extra fabric is folded up under the obi for a smoother, more formal fit. The shorter length is more popular with my younger Japanese friends but that might not be an accurate thing to say about length preference in general^^
Men's yukata sleeves are sewn to the body almost all the way down to the waist, and have sharp corners on their sleeves. Since men's obi are almost half as wide as woman's and are worn lower in the body, there's no need to adjust the fabric around the obi.
Unisex yukata are about halfway between the two. They are usually seen at hot springs and are usually blue or the hot spring's colors.
There is also 'western' style yukata, which do not have the cool sleeves we think of kimono as having. These look like cotton European bathrobes, and aren't particularly popular anywhere any more as far as I know.