七月流火

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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.

In the seventh month, the Fire Star passes the meridian; in the ninth month, clothes are given out.
In the days of first month, the wind blows cold; in the days of 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