The only free and comprehensive online etymological dictionary of the Spanish language
julio (1) m. (Noun) "July"
13th cent. From Latin Iulius 'id.,' taken from the name of Julius Caesar in honor of his birth month. Iulius was not Julius Caesar's first name but his clan name. Of uncertain origin. According to Pokorny (1959), from an unattested Old Latin form *Iovilios "Jove's;" a clan name expressing special attention to the deity Jove.
Portuguese julho, Galician xullo, Catalan juliol, French julliet, Italian luglio