Being that it's the middle of July and super hot here in Kyoto it would seem obvious that it's already summer, but to me I only really know its summer in Japan when the festivals start. Recently I went to the Tanabata Festival in neighboring Kizugawa City.
To me it really feels like summer when I can go to a festival where the streets are lined with vendors selling all sorts of typical Japanese foods like, okonomiyaki, yakisoba, karage and other such treats. The day of the festival was pretty hot and humid and with so many people packed into the streets it only increased the heat. But its so nice to walk down the rows of food vendors smelling the different foods and maybe drinking a cold beer. The best part of the whole thing is that most of the young children were dressed up in colorful yukata (a light summer version of the traditional kimono). At the festival there were large decorations were people had written down their wishes for the future (see above picture), one such wish was for world peace.
With the smell of grilled foods and tons of people out creating a lively atmosphere it really made me realize how much fun summer in Japan can be despite the oppresive humidity.