They provide an economy of expression that is especially valuable in the very short haiku, as well as the longer linked-verse forms renku and renga, to indicate the season referenced in the poem or stanza.
For spring, these would be: Japanese haiku poets often use a saijiki, a book like a dictionary or almanac for kigo.
Modern saijiki and kiyose are divided into the four seasons and New Year, with some containing a further section for seasonless (muki) topics.
The traditional date of Tanabata is 7th day of the 7th month of the Japanese calendar, which falls in early Autumn.
The modern use of the Gregorian one has moved the observance to 7 July, which has resulted in a dispute as to whether Tanabata should be treated as a summer kigo.