In hokku the concept of harmony is very important. If a verse is composed of elements that are inharmonious with one another, the hokku will fail. But beyond that, the hokku should be in harmony with the season in which it is written.
It often seems initially odd to many Westerners that one should read a hokku in the season in which it is written, but it really is not an unfamiliar concept. If we see a house with Christmas lights still up in August, we feel there is something out of place; and if we see a pottery Halloween pumpkin in May, we have the same feeling of disharmony.
It is the same with hokku, only we become even more aware of such discords of object and time, because hokku takes us away from our personal and social preoccupations and puts us in touch with the seasons that were for millennia the essential and unfailing context of our ancestors’ lives.
This is not something peculiar to hokku. It is a commplace in the aesthetics of the culture out of which hokku grew. And as R. H. Blyth reminds us, the contemplative arts of Japan share as their foundation virtually the same aesthetic principles, so that if you understand one, you understand them all. That is why, on entering a traditional Japanese home, one will find a flower arrangement in harmony with the present season; and if there is a hanging scroll, it will depict a scene in harmony with the season.
To have an arrangement of lilies in midwinter, or of daffodils in autumn, is discordant — inharmonious. And in hokku one is very sensitive to such things, because hokku is to put us in harmony with Nature, not to divide us from it.
That is why in hokku we both read and write verses in season. It is true that we will sometimes use a verse from one season in discussion during a different season, but that is merely for purposes of learning and explanation, and it does not in any way negate the principle that the hokku and the season should be in harmony when written and when read.