Albert Barnes Commentary Proverbs 26:1

"As snow in summer, and as rain in harvest, So honor is not seemly for a fool." — Proverbs 26:1 (ASV)

In Palestine, there is commonly hardly any rain from the early showers of spring to October. Therefore, “rain in harvest” sometimes became (see the marginal reference) a supernatural sign, sometimes, as in this case, a proverb for anything that was strange and incongruous.