七月流火

From the Classic of Poetry, poem 154 (《詩經·豳風·七月》):

七月流火，九月授衣。
一之日觱發，二之日栗烈。 [Pre-Classical Chinese, trad.]

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七月流火，九月授衣。
一之日觱发，二之日栗烈。 [Pre-Classical Chinese, simp.]

From: The Classic of Poetry, c. 11^th – 7^th centuries BCE, translated based on James Legge's version

Qīyuè liú huǒ, jiǔyuè shòu yī.
Yī zhī rì bìfā, èr zhī rì lìliè. [Pinyin]

In the seventh month, the Fire Star passes the meridian; in the ninth month, clothes are given out.
In the days of [our] first month, the wind blows cold; in the days of [our] second, the air is cold.

In antiquity, Chinese astronomers saw the westerly motion of Antares, the Great Fire Star (大火), as a sign of autumn's arrival. Whether this idiom should be interpreted as referring to the current heat or the imminent cooling is a matter of constant dispute.

七月流火

1. the summer heat recedes during the seventh lunar month, and the cold season becomes imminent
2. scorching heat during July