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Derivation: This is another of Herbert's numerous Arabic/Islamic borrowings in Dune. Ramadhan is from Ramadan, the month in the Hijri calendar devoted to fasting and charitable giving. It is the ninth month of this strictly lunar calendar.

A lunar year is about 11 days shorter than a solar year, so the month of Ramadan moves backward through the seasons. But that's just Earth; on the Fremen's world there may have been no discrepancy.