What is with kimono that makes Japanese guys to fall for the girls in many anime? I've been watching lots of anime series and in lots of them the guy realizes that he loves the girl when he sees her in a kimono specially for the festivals. Just this last week it was both in ReLIFE and Twin Star Exorcists. Just what is with that?
It's a similar trope to one you see in Western media, where the normally un-feminine (or dorky, or nerdy, or otherwise not stereotypically attractive) female character dresses up for some kind of fancy party - if the characters are school age, then it's frequently a prom. So they ditch the t-shirt, jeans and sneakers, don a dress and heels, swap the glasses for contacts, maybe put on makeup and get their hair done, and then become the belle of the ball and make the male protagonist swoon.

A kimono (or yukata, since a lot of these scenes take place at festivals in the warmer half of the year) is a very traditional dress, and it's associated with elegance given its fabric (often silk) and intricate patterns. In addition, when wearing one, women will often put their hair up, maybe put on a little makeup, and wear geta which are elevated sandals, which have an effect on the wearer's height and posture similar to wearing high heels.