الميدى الابيض ينفع في النهار الاسود
El-midi el-abyad yenfaa fil nahar el-eswed
Translation: The white midi would be of value on a black day.
Meaning: Save for a rainy day. There is not much known about the midi, a coin that came into circulation around Ottoman times and disappeared in the nineteenth century. In current parlance, you'll hear the same saying with the word "qersh" replacing the midi. A midi, a fraction of a French franc, could buy a meal perhaps.
Source: ُEgyptian proverbs, (Al-Ahram Center For Translation and Publishing, 1986) by Ahmed Taymour Pasha,
Translated by: Nabil Shawkat