May is called season of “obitsuki”. Obitsuki means to show obi without wearing haori. Recently in March and April, we don’t need outer wear in many days, but in the past it had been custom to wear something until May.
Of course, you can wear some outfit in May. If we wear expensive obi in particular, we wear thin coat called “chiriyoke” to prevent the kimono get dirty when we walk.
When we enter a house, it is good to take off outer wear as season of obitsuki. We should not wear haori which has a custom not to take off in the house.
In May in some years, it is hot enough to have sweat. On very hot day, you can wear hitoe, which is earlier than usual.
Recently at the end of May, most people wear hitoe.
From May, design which puts emphasis on summer has been used often. Iris ensata var and iris sanguinea are popular, but swallow are hydrangea, which are kind of early a little, start to be used in this month. It is a month when coolness is liked as color.