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.