For most of Roman history the month was simply Quintilis, the fifth month of the year that began in March, its numbered name of a piece with September through December. In 44 BCE, in the last months of Julius Caesar's life, the month of his birth was renamed Iulius in his honour, the first time Rome had given a month to a man rather than a god or a number. The gesture belonged to the same accumulation of divine honours that ended with Caesar's formal deification two years after his assassination, and it set the precedent that gave the following month to Augustus a generation later. Later emperors tried to repeat the trick, renaming months for themselves and their families, and every one of those names died with its author; only Caesar's and Augustus's held.
The Roman religious calendar of the month was dominated by the Ludi Apollinares, the games of Apollo, instituted in 212 BCE during the darkest years of the war with Hannibal after a prophecy promised victory if the god were honoured, and made annual soon after. They ran from the sixth to the thirteenth of July. The Neptunalia of the twenty third, kept in the midsummer heat with arbours of branches for shade, asked the god of waters for relief in the driest stretch of the Italian year, and the obscure Furrinalia followed two days later.
The renaming survived the fall of the religion that produced it: the Christian centuries kept Caesar's month without embarrassment, and every language of Europe still carries his name each summer.