What are the meaning and the usage of 夏に入{い}る and 立夏?


[According to
weblio.jp](https://www.weblio.jp/content/%E5%A4%8F%E3%81%AB%E5%85%A5%E3%82%8B),
夏に入る is a synonym of [立夏](https://jisho.org/search/%E7%AB%8B%E5%A4%8F). I have
two questions regarding these words.

  1. Do these 夏に入る and 立夏 refer to a specific day, or they just mean the beginning of summer in a broader sense?

I could only find examples of 夏に入る used within a 俳句. Are 夏に入る and 立夏 used only
in poetry, or do they have other usages beyond that?




立夏 is one of the 24 words called
[二十四節気](https://ja.wikipedia.org/wiki/%E4%BA%8C%E5%8D%81%E5%9B%9B%E7%AF%80%E6%B0%97)
([Solar term](https://en.wikipedia.org/wiki/Solar_term)). It specifically
refers to May 5 or May 6 (depending on the year; see the linked article).
Despite its appearance, it's not a word that simply refers to the start of the
summer. Some Japanese seasonal events including
[_setsubun_](https://en.wikipedia.org/wiki/Setsubun) are based on 二十四節気.

二十四節気 was defined many years ago based on the climate of a certain region of
China, and the terms generally do not match how Japanese people usually sense
seasons today. Most Japanese people think May 6 is still spring even though 立夏
is technically the start of the summer. The start of the spring (立春) according
to this system is February 4, but this is usually the coldest time of a year
in Japan. So 立春, 立夏, etc. are not very useful concepts in our daily life.

> Solar terms originated in China, then spread to Korea, Vietnam, and Japan,
> countries in the East Asian cultural sphere. Although each term was named
> based on the seasonal changes of climate in North China Plain, peoples
> living in the different climates still use it with no changes.

However, haiku is one of the fields where those 24 terms and the sense of
seasons based on them have traditionally been considered important. If you
know the _kigo_ system, it still respects this 二十四節気 system [as explained
here](https://en.wikipedia.org/wiki/Kigo#Japanese_seasons), and that's why 立夏
is a common word in traditional haiku. 夏に入る is an wago-paraphrased version of
立夏 in haiku, and it typically refers to May 5-6, too.

In daily life, people simply say 夏が始まる, and that's normally no sooner than
June 1.




立夏 is one of the 24 words called
[二十四節気](https://ja.wikipedia.org/wiki/%E4%BA%8C%E5%8D%81%E5%9B%9B%E7%AF%80%E6%B0%97)
([Solar term](https://en.wikipedia.org/wiki/Solar_term)). It specifically
refers to May 5 or May 6 (depending on the year; see the linked article).
Despite its appearance, it's not a word that simply refers to the start of the
summer. Some Japanese seasonal events including
[_setsubun_](https://en.wikipedia.org/wiki/Setsubun) are based on 二十四節気.

二十四節気 was defined many years ago based on the climate of a certain region of
China, and the terms generally do not match how Japanese people usually sense
seasons today. Most Japanese people think May 6 is still spring even though 立夏
is technically the start of the summer. The start of the spring (立春) according
to this system is February 4, but this is usually the coldest time of a year
in Japan. So 立春, 立夏, etc. are not very useful concepts in our daily life.

> Solar terms originated in China, then spread to Korea, Vietnam, and Japan,
> countries in the East Asian cultural sphere. Although each term was named
> based on the seasonal changes of climate in North China Plain, peoples
> living in the different climates still use it with no changes.

However, haiku is one of the fields where those 24 terms and the sense of
seasons based on them have traditionally been considered important. If you
know the _kigo_ system, it still respects this 二十四節気 system [as explained
here](https://en.wikipedia.org/wiki/Kigo#Japanese_seasons), and that's why 立夏
is a common word in traditional haiku. 夏に入る is an wago-paraphrased version of
立夏 in haiku, and it typically refers to May 5-6, too.

In daily life, people simply say 夏が始まる, and that's normally no sooner than
June 1.

