Kigo (Season Words) (季語)
It is true, however, that some haiku poems have no season word and, in such cases, those haiku poems were referred to as 'zo no ku' (miscellaneous poems) in hokku (the first line of waka or haiku) of haikai in the Edo period which became known as mukihaiki (haiku without season word) in the modern haiku after the generation of Shiki MASAOKA.
As a general rule, a season word is included in a haiku. When a haiku contains 2 or more season words, it is referred to as kigasane (having more than one season phrase in a haiku) and, in such cases, the season word is the one which is in keeping with the topic.
The Roles of Season Words
Season words play a major role in haiku. Season words convey the reader the image symbolizing the gist of poetic sentiment. It can also be called a suggestion. Season words also make it possible to express time and space as one dimension.
Season Words in Senryu (17-syllable satirical poems)
Although its format is similar to haiku being a 17-syllable poem, since senyu emphasizes irony, satire, humor and human foibles, season words may not play a significant role.
Saijiki (a catalog of season-specific words used in composing haiku)
Collection of season words is referred to as Kiyose (a practical catalog of season-specific words used in haiku) or Saijiki and a variety of these catalogs have been published. It can be said that Saijiki is the summarization of poetic dictions.
Most of Saijiki conform to Nijushisekki (24 divisions of the old calendar) to correspond season words to seasons. Words associated with the new year used in the old calendar days were classified as season words for spring but, since the calendar reform in Meiji, the new year changed to the winter time whereby a separate section for 'new year' in addition to the four seasons was established.