Our calendar is a Roman calendar. Our months are Roman names. The days in English are taken from Norse folklore, but the names in romance languages derive from the Roman. The Roman calendar derived from the Greek calendar and from the Etruscan dynasty.
The original Roman calendar began in March and until 153 B.C. had 10 months, when two were added. Four months are named for numbers, those numbers making sense when you start with March. “Calendar” comes from “calends,” the first day of the month in the Roman calendar and the day of the new moon and comes from calare, “to proclaim,” from the practice of calling people together on the first of the month to proclaim the time of the new moon, the day of the Nones, and festivals and sacred days. Officials called pontifices watched for the new moon and proclaimed the number of days until the Nones.
Nones (nonae) were the ninth day inclusive before Ides (idus). Ides were the day of the full moon and were the 15th of March, May, July, and October and the 13 of other months.
Other days were named by their relationship to the Kalends, Ides, or Nones. For example, Aug. 30 would be ante diem ii kalend September, or two days before the Kalends of September, which would read a.d.ii kal.Sept.
Roman mensis (months) are, followed by their French equivalents, which are not capitalized, to give an idea of a romance derivation and because I know French:
Mars: Martius, named for Mars, the god of war; Fr. mars.
April: Aprilis, probably from Latin meaning to open; Fr. avril.
May: Maius, from Maia, goddess of growth and increase; Fr. mai.
June: Iunius, perhaps from Juno, goddess of marriage and well-being of women; although “Brewer’s Dictionary of Phrase and Fable” said it comes from the Roman clan name Junius, which is akin to juvenis, which means young; Fr. juin.
July: Quintilis, the fifth month, later called Julius, named by Mark Anthony in honor of Julius Caesar. “July” was accented on the first syllable until the 18th century; Fr. juillet.
August: Sextilis, the sixth month, later Augustus, renamed in 8 BC in honor of the first Roman emperor. It was his lucky month; Fr. août.
September: from septem, seven; Fr. septembre.
October: from octo, eight; Fr. octobre.
November: from novem, nine; Fr. novembre.
December: from decem, ten; Fr. décembre.
January: Januarius, named for the god Janus, who presided over the entrance to the year and had two faces, one looking back and one looking forward; Fr. janvier.
February: Februarius: Latin februum, meaning purgation, from the Roman month of purification; Fr. février.
Dies Solis or Dominica: Sunday, Sol’s Day; Fr. dimanche.
Dies Lunae: Monday, Moon’s Day; Fr. lundi.
Dies Martis: Tuesday, named for Mars; Fr. mardi.
Dies Mercurii: Wednesday, named for the god Mercury; Fr. mercredi.
Dies Jovis: Thursday, named for Jove, or Jupiter; Fr. jeudi.
Dies Veneris: Friday, named for the goddess Venus; Fr. vendredi.
Dies Saturni: Saturday, named for Saturn; Fr. samedi.
Julius Caesar reformed the calendar to realign it with the sun, adding 67 days between November and December in 46 B.C., that year thus containing 445 days including Caesar’s intercalation (the insertion of a day or days in a calendar, as we do with Leap Day). The Julian Calendar was used until Pope Gregory XIII instituted the Gregorian or New Style Calendar in 1582 to further correct the calendar, using a system developed by Aloysius Lilius of Naples. Gregory XIII declared that Oct. 5 that year would be Oct. 15. Protestants resisted this change because it was wrought by a Catholic, the British people holding out until 1752, when the day following Sept. 2 was declared to be Sept. 14, causing riots because people thought they had been robbed of eleven days. For this reason histories and documents will sometimes say “Old Style” or “New Style,” and I seem to remember reading once about a problem with the exact date of George Washington’s reaching maturity because he was born in 1732 and turned 21 after the calendar was changed.
One last note: Janus led to the words Janus-faced, meaning two-faced or duplicitous; Janiform, meaning have a face on two sides, as a Janiform coin; and a Janus-faced lock, a lock that can be opened with a key from either side.
Sources: Encyclopaedia Britannica, Brewer’s Dictionary of Phrase and Fable, and The Old Farmer’s Almanac