Rent a Yukata

Hi there

I'm heading over to Japan the third time in may 2018, but the first time with my girlfriend.

I'd like to surprise her and go on a walk with her through Kyoto with a Yukata on.

So my question is:
Can you recommend an "easy-to-find" and english-guided Yukata rent store in Gion area (or around there)?
We would like to rent them for a day and then bring them back.
How long in advance shall I make reservations? It's still three months until we go there... should I just ask them by mail now?

In May is not a season to wear a Yukata outside, you had better rent a normal Kimono.
Rent shops with English pages can be easily found in Google.

Do you understand the distinction between "Yukata" and regular "Kimono"?

However, you can wear it in any season in the spa(onsen=hot springs) town.
Yukata in the spa town and other Yukata which you wear
at the festival or in the regular town etc are the same name, but different.

I purchased several yukatas and obi on various trips (mostly springtime) from yukata shops in Tokyo. I would be surprised if you could not buy one in Kyoto. Why rent? Then you have a souvenir that you can wear at home. The Kyoto Handicraft Center (located very close to Heian Shrine) sells them:

http://www.kyotohandicraftcenter.com/shop_guide/detail.html?id=24

I'm assuming the OP meant to rent a kimono, in that case, google 'maiko for a day' in Kyoto.

Yukata are not expensive. You can go into many kimono stores and buy a yukata, even in the off season, for anywhere from 2,500 to 10,000 yen for a full yukata set. I know because I have done it to buy gifts for people overseas. Some kimono shop owners may grumble because they've put their summer yukata in winter storage and it's a hassle to go get the yukata out. I've experienced that reaction. But if you go into a few places, you can certainly find a yukata this time of year. Stay away from department stores, though. They usually send their stock back to a warehouse, so they probably won't have any yukata in the winter.

You can also shop on Amazon.co.jp and find yukata anytime. And delivery in Japan is often the next day.

As another person pointed out, you may mean "kimono," not "yukata."