There is a common myth that Julius Caesar and Octavian Augustus added the months of July and August, respectively. But there’s a reason that myth never makes an appearance in any popular culture surrounding the two men: because it’s a myth.
Well, it’s not entirely a myth. But I will get into that later. You see, the months that were added to the calendar that we use today were actually January and February, with February originally being placed at the end of the month. This is thanks to the Roman ruler Numa Pompilius (a Roman king), who decided to make the calendar longer after realizing that the calendar was far too short to make up for the actual year. This comes as a result of the original Roman calendar, which was merely taken from it’s lunar Greek predecessor, was only 304 days long. When Numa added the two months, the calendar got pushed up to 355 days, eliminating the need for adding an extra month(s) just to play catch-up to the rotation of the sun.
So, if January and February were actually the months that were added, where did July and August come from? Well, Before the dictator and emperor came in, the calendar looked like this: January, Martius, Aprilis, Maius, Juniius, Quintilis, Sextilis, September, October, November, December, and February. Can you see which ones were changed? Well, Quintilis wouldn’t be changed until after Julius Caesar’s death. As a part of his divination by the people, Augustus changed the month of Quintilis (which means fifth in Latin) to July, in honor of Caesar’s birth month.
August, however, would be implemented while Augustus was still emperor, with Sextilis being the one that gets renamed. This is in honor of his good emperorship in 8 BC, presented by the Senate. His birth month is not in August, however, but rather September. So why did Sextilis get changed? Possibly to show the sign of lineage. Augustus wanted the month to follow July, as a show of his relation to the popular dictator (he was adopted by Julius Caesar). Plus, since both July and August had 31 days, it would also show his equal greatness to his predecessor.
So, why the popular myth, then? Most likely as a show of the two men’s inflated egos, or even because they were the only two months whose names were changed. No other month would have their name changed (although February would find itself moved to be between January and Martius in the fifth century). Either theory would make sense, although it is difficult to say how inflated the men’s egos were. They were certainly grand, although that might just be because they were Romans (the Romans were quite extravagant and egotistical normally). So, I can’t really say why the myth exists, or why it persists.