The third of the twenty-four nijuushi sekki, Keichitsu, means "Awakening Insects" or 'startling insects'. The 24 sekku are imported from China. During this period, many thunderstorms are said to occur due to the shift from cold weather to warm, which startles hibernating insects from the ground. I suppose it is more accurate to mark the season by the insects one would see vs. simply saying "it's a stormy season", as many seasons include different types of storms.
Japanese beetles are amongst the list of hibernating insects, as well as some grubs, cicadas, and grasshoppers. But really, the season doesn't only refer to insects- rather, the imagery can mean any creature living in the ground who would then have to come out of the soaked earth. Supposedly, snakes and frogs will be seen more often during this time, probably because acquiring food is now easier.
In related news, Japanese people are freaking SMART. I had no idea what the woven reed mats around trees were in photos, but thanks to this blog, I do now! They protect burrowing insects and trees in the winter! How awesome! <3 This time of year, the mats are taken off and burned. So, trees are protected from insects because the insects burrow into the mats instead of the bark for protection (keeping the tree safe as well), they keep trees insulated from the freezing winter, AND the mats make excellent firewood in the last days of the cold weather!
On this day, I would probably wear a wool kimono in deep blues or browns/greens. Browns and greens for earth and insects, indigo for the cold weather or storm/water references, and double points if you have an obi with a grasshopper or cicida motif! (By the way, congrats to whomever won the snake juban on Ebay. I'm so envious! What a rare and beautiful piece!)