In their incredibly informative etymological whiteboard series for Mental Floss , linguist Arika Okrent and illustrator Sean O’Neill verbally and visually explain the sources for which months on the English calendar were named. The idea was a bit rough at first, but it all smoothed out in the end.
…in 46 BC Julius Caesar introduced a new system, with a pretty consistent year: 365 days, with an extra day added to February every four years. This is when Quintilis, the 5th month, was renamed Julius, July, the month of Caesar’s birthday. Augustus the emperor of Rome who came after Caesar, thought this month naming gig was a fine idea, and so Sextilis became Augustus, August for him. The fashion for month naming ended there, so from September on, we were left with the old numbered system. But because the beginning of the year was moved from March to January the numbers didn’t fit anymore. The 7th month became the 9th month, and the 10th a 12th.
In 2015, Okrent and O’Neill offered a similar explanation for the days of the week.
Our weekday names start in ancient times with the astrological idea that the planets rule the hours of the day. In that outdated concept of the universe there were 7 planets going out from Earth in order of distance: moon, mercury, venus, sun, mars, Jupiter, Saturn. Going in towards Earth, the 1st hour of the day was ruled by Saturn, then Jupiter, Mars, sun, and so on cycling through those over and over through the hours. Each day was seen as ruled by whatever its first hour was ruled by. So Saturn had the first hour, but then 24 hours later, at the 25th hour, the count was on Sun, then 24 hours after that Moon. That’s how we got the order Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus.