In Arabic documents issued by the dīwān of the Norman kings of Sicily during the twelfth century, brutuyūn and istiriyūn mean, respectively, ‘June’ and ‘July’. The geographer al-Idrīsī, who completed the Kitāb nuzhat al-mushtāq in Palermo in 1154, also uses istiriyūn for ‘July’. These month-names are derived from Greek *Πρωτοϊούνης, literally ‘first June’, i.e. June, and *Ύστεροϊούνης, literally ‘second June’, i.e. July. The linguistic circumstances in which the coining may have occurred are discussed.