The origins of the names of the months and the days

from the series of short articles about calendars

By legend, the Roman calendar (our calendar) was created by the mythic first king of Rome, Romulus, when he founded the city in 753 BC. But unlike most moon-based calendars, Romulus, for some unknown reason, concocted a year composed of only 10 months, not 12. Romulus's infatuation with ten extended to naming his months. The first 4 months he named Martis for the god of war; Aprilis, which probably refers to raising hogs; Maius, for a local Italian goddess, and Junius, for goddess Juno. Then he simply fell into counting the months, naming from the fifth, sixth, seventh, eighth, ninth and tenth: in Latin Quintilis, Sextilis, September, October, November and December. This explains for why the ninth, tenth, eleventh and twelfth months of our modern calendar are still numbered in Latin as the seventh, eighth, ninth and tenth months. Later, Quintilis was replaced by Julius for Julian Caesar and Sextilis by Augustus for Emperor Augustus.
The order of the day names themselves comes from ancient Mesopotamian astrologers' attaching a planet-god to preside over each hour of the day, arranged according to their correct cosmological order. For instance Saturn controlled the first hour of Saturn's day (Saturday), followed in its second hour by Jupiter, then by Mars, the Sun, Venus, Mercury and the Moon. In the eighth hour the cycle started again with Saturn, and the progression repeated until the twenty-fourth hour of the day, which happened to fall to Mars. Because the next hour in the cycle–the first hour of the new day belonged to the Sun god, the day after Saturday was called Sunday.
The ancients used a simple device for keeping track of the proper names of the hours and days in relation to the planet gods. They used a seven-sided figure, with each vertex marked with a planet's name in the proper order. Archaeologists found one of these wheels drawn as graffiti on a wall when they excavated Pompeii, like the above figure. Following the arcs of the figure, one can find the established order of the day names: Day of Sol, day of Luna, day of Mars, day of Mercurius, day of Jupiter, day of Venus, day of Saturnus.
Indeed, the seven-day system was already ancient by Constantine's day. It seems to have originated circa 700 BC in Babylon, when astrologers assigned their planet-gods to the days of the week–names the Romans replaced with their own planet-gods. For instance, the day of Nabu, the Babylonian god of the scribes, became in Latin the day of Mercurius, the Roman god of communication and today survives as Mercredi in France. In English, however, the day of Nabu is known as Wednesday because of a curious twist of history: the fact that the seven-day week did not penetrate to Britain until the era of the Anglo-Saxon conquests in the fifth century. At that time the invaders wanted to take on certain Roman trappings but clung to their own pagan religion and gods. So Nabu in Babylon became Mercurius in Rome and Woden the German (and Viking) god of poetry in Britain. Also the Anglo-Saxonic god of war Tiw is honored in Britain instead of Mars (Tuesday), the god of thunders Thor instead of Jupiter (Thursday) and the goddess of love Freyja instead of Venus (Friday). Besides, the Anglo-Saxonic goddess Eostre gave her name to the Christian feast of Easter.
Additionally, Saxons gave us the English word 'day', which comes not from the Latin 'dies' but the word in Saxon for 'to burn', during the hot days of summer.

(information retrieved from the book of David Ewing Duncan, 'The Calendar')