Ancient Mythology of The Calendar

January is the first month of the Roman Calendar year.
~ it's original Roman name, Ianuarius, is named after the god Janus in Roman mythology. Janus, was the god of the doorway, a god of new beginnings, and had two faces to see in opposite directions; symbolizing the year past and the coming year ahead

The Saxon term was Wulf-monath (meaning wolf month)
Charlemagne's named it Wintarmanoth (winter / cold month).

February is the second month of the Roman Calendar year.
This is the month of purification. A Roman purification festival was celebrated on February 15, called Februa and so this month came to be known as Februa's month.

The day before that, and the day after (ides), was a holiday to honor Juno. The goddess Juno was the Queen of the Roman gods and goddesses, and also the goddess of women and marriage.

The Anglo-Saxon term was Solmoneth (mud month)

March is the third month of the Roman Calendar year.
It is the month named after the planetary god of war, Mars. This month typically signaled the start of battles after the winter months. Mars equates to the Greek god Ares, or Aries. Aries is the first sign of the zodiac, which begins at the spring equinox.

Saxon named it Lenctmonat (the equinox and eventual lengthening of days ~ eventual namesake of Lent)

April is the fourth month of the Roman Calendar year.
The month of April begins with the Latin prefix Ap, or ad ; implying to, opening, as so many buds and blossoms open in this month. Considered a sacred Roman month for the goddess Venus. The name 'April' is probably from Apru, an Etruscan borrowing of Greek Aphrodite, a fertility goddess.

The Anglo-Saxons called April Oster-monath or Eostur-monath, the period sacred to Eostre or Ostara, the pagan Saxon goddess of spring, from whose name is derived the modern Easter.

May is the fifth month of the Roman Calendar year.
May was named after the goddess of growth, Maia, and heralded the bloom of springtime plants and hence the tradition of maypole dances. She was wife of Vulcan, mother of Mercury by Jupiter and the daughter of Atlas.

In the neopagan Wheel of the Year, May begins on Beltane in the northern hemisphere and Samhain in the southern hemisphere.
In the Irish calendar May 1 is Beltane (Bealtaine), the first day of Summer.

June is the sixth month of the Roman Calendar year.
June named after the Roman Juno, who was the Queen of the gods and wife of Jupiter.

The summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere occurs on dates varying from 20 June to 22 June. In the pagan wheel of the year the summer solstice is the time of Litha and the winter solstice is that of Yule.

In the Irish Calendar the month is called Meitheamh and is the middle month of the summer season.

July is the seventh month of the Roman Calendar year.
It was inserted by Julius Caesar under the original Julian Calendar.

In the Irish Calendar the month is called Iúil and is the third and last month of the Summer season.

August is the eighth month of the Roman Calendar year.
It was inserted by Augustus Caesar, the adopted heir of Julius Caesar, under the modified Julian Calendar.

In Irish, it is known as Lúnasa, a modern rendition of Lughnasadh, from the god Lugh.
August 1, Lá Lúnasa in the Irish Calendar, is still regarded as the first day of Autumn.

September was the seventh month of the former Roman Calendar year.
Sept for seven. September was changed to the ninth month of the Julian Calendar year following the additions of July and August.

October was the eighth month of the former Roman Calendar year.
Octo- for eight. October became the tenth month of the Julian Calendar year following the additions of July and August.

In Scottish Gaelic, it is called an Damhar, meaning "rutting time" (of stags).
In Irish, October is called Deireadh Fómhair, meaning "end of harvest-time".

November was the ninth month of the former Roman Calendar year. In Latin, novem means "nine".
November became the eleventh month of Julian Calendar year following the additions of July and August.

In the pagan wheel of the year, November begins at or near Samhain in the northern hemisphere and Beltaine in the southern hemisphere.

December was the tenth month of the former Roman Calendar year.
Dec- for ten. December became the twelfth month of the Julian Calendar year following the additions of July and August.

In Irish, December is called Mí na Nollaig, meaning "month of Christmas".

In 1582, Pope Gregory XIII changed the calendar system to establish the Gregorian calendar, with January as the first month of the year.