My friends and I are planning to attend a natsu matsuri, a summer festival, in Japan. We all have bought our yukatas online, and unfortunately two of our yukatas are only knee-length long. Is this still appropriate to wear to the said event, or are we in any way offending Japanese people?
Did your yukata come with shorts (short pants)? If so, there’s no problem, just wear those.
Remember that yukata are effectively pajama and no one wants to really see your underwear or worry that you’re going “commando” (no underwear or undergarments under your yukata). This is true for both women and men. It’s very easy for yukata to become accidentally undone even if it’s well tied off.
If yours didn’t come with shorts, you can wear some other pair of shorts, bike shorts, yoga pants, tights, or something else that you have.
TLDR; foreigners can get away with a lot but don’t make people worried you’re commando underneath the yukata.
It's not offensive, it's just a bit silly-looking. Summer festivals are very casual events, so friends may rib you a bit but nobody will seriously mind, especially after a couple of beers.
As an inexact analogy, imagine wearing a pair of jeans that fit otherwise but end mid-calf: only the fashion police would take offense (and who knows, maybe Capri pants are back in style this year).
I do second RoboKaren's advice to wear underwear. Even a full-length yukata can slip pretty easily when sitting down.
If I were you, I would not wear the knee-length yukata, and would just go to the festival in normal clothes.
No, you are not going to offend anyone; if there's one area where Japanese society is tolerant, it's clothing (at least outside of school or business settings). At the same time, I still would call it inappropriate, not in the sense of "offensive", but in the etymological sense of "not suited". As you can easily see from a Google Images search, and will definitely see when you are there, it's just not done. I am, in fact, quite surprised that you were able to buy one, and would be interested to know where you bought it (I suppose, from a retailer with only cursory knowledge of what they are doing).