Kigo
A kigo is a word or phrase used in Japanese poetry that evokes a a particular season. Usually, any mention or Shallow Parody of haiku (or Japanese poetry as a whole) will very likely mention cherry blossoms. We'll leave it to The Other Wiki to tell us about the significance of Cherry Blossoms [1] (sakura) to Japanese culture, because the point here is that they only appear in spring. Mentioning them is a economical way of setting the season, and haiku is all about economy of expression.
Other common kigo are:
Spring: Warm (atatakashi or nurumu), the warming weather; uguisu, Japanese bush warbler, regarded as a harbinger of spring; Frogs: especially their calling
Summer: Hot/heat; Rainy season starting in mid-June; Wisteria which blooms in early summer and lotus in late summer.
Autumn (Fall): Coloured leaves; Typhoon, typhoon season is roughly June-December; Insects, implying singing insects such as crickets.
Winter: Snow, Cold, fugu soup, a seasonal dish, Christmas (modern).
- ↑ Actually, we don't have to, because the wiki has its own perfectly good page on the subject. God, we're good, aren't we?