Usui is the second term of the solar calendar in the Japanese old traditional calendar.'Usui' refers to wateriness, or in this case, rain water. This does not necessarily mean that it is the rainy season in Japan, since the calendar is Chinese in origin and was imported by the relatively newly-established Japanese society.
Remember that China and Japan once used the same lunisolar system, with both a lunar cycle set and a solar cycle set. The nijuushi sekki are the 24 sets of seasons, which are further divided by 3 for a total of 72 seasons. Some 'seasons' last only a day, especially for special solar/lunar events. Those who pay very close attention to this calendar may develop a more keen eye when looking at what someone is wearing for a special event, such as festival clothing that only comes out once a year!
Perhaps you could go one of two ways with clothing today. Either dress in bright, popping colours to remind people of the coming dry weather full of flowers and hues, or go with the weather and wear water-themed or cool-coloured clothing that matches your environment, such as grey, purple, and blue kimono on a dreary day.