Manning, To my knowledge, there was no day named Monday... or any other name in Biblical times. In Genesis, the days of the week are numbered, but not named, even the meaning of Sabbath (Shabbat ) is seventh, the seventh day of the week.

The Hebrew calendar is based on the lunar cycle. It has 12 months, each with 29 or 30 days. Every four years, there are leap years in which an extra day is added to February and/or January.

God would not have used a calendar where each day of the week is named for a pagan deity. After writing all that follows below, I believe God would still be content with His original phrasing seen throughout the Tanakh/Old Testament: "Now, on the third day of the sixth month...."

Today, in the international standard ISO 8601, Monday (named after the Astrological Moon goddess), is treated as the first day of the week, but in the US, Canada, and Japan, it's counted as the second day of the week. Monday comes after Sunday as the 2nd day of the week in the 16th century Gregorian Calendar in use today.

The creation is recorded in Genesis 1- 2. Most of God’s creative work is done by speaking, displaying the awesome power and authority of His Word. God’s creative workweek is described:

Creation Day 1 (Genesis 1:1–5)

God created the heavens - everything beyond the earth, outer space, and the earth, made but not formed in any specific way, although water is present. God speaks light into existence, separating the light from the darkness, naming the light "day" and the dark "night."

Creation Day 2 (Genesis 1:6–8)

God creates the sky, as a barrier between water upon the earth's surface and the moisture in the air. This gives the earth an atmosphere.

Creation Day 3 (Genesis 1:9–13)

God creates dry land - Continents and islands above the water. The large bodies of water are named "seas" and the ground is called "land." God declares that all this to be good.

God creates all plant life both large and small. He creates this life to be self-sustaining; plants can reproduce. The plants were created in great diversity (many “kinds”). The earth was green and teeming with plant life. God declares that this work is also good.

Creation Day 4 (Genesis 1:14–19)

God creates all the stars and heavenly bodies, their movement will help man track time. Two great heavenly bodies are made to light the earth. The sun, the source of light, and the moon, to reflect the sun's light by night. Their movement will distinguish day from night. Again, this work is declared to be good by God.

Creation Day 5 (Genesis 1:20–23)

God creates all life that lives in the water. God also makes all the birds, and flying insects as well; unless they were made on Day 6. All are made with the ability to reproduce their species. The creatures made on Day 5 are the first blessed by God. God declares this work good.

Creation Day 6 (Genesis 1:24–31)

God creates all the creatures that live upon dry land, including every type of creature not previously included, and He creates man. God declares this work good.

When God was creating man, He took counsel with Himself. "God said, ‘Let us make man in our image, in our likeness'" (Genesis 1:26). This is part of the foundation for the Trinity, as God reveals an "us" within the Godhead. God creates man in the image of God (men and women both), - special above all other creatures. God places man in authority over the earth and over all other creatures. God blesses man, commanding him to reproduce, fill the earth, and subdue it (under stewardship as authorized by God). God announces that man and all other creatures are to eat only plants. God will rescind this dietary restriction in Genesis 9:3–4, after the flood.

God’s creative work is complete at the end of the sixth day. The entire universe was fully formed in these six periods labeled as "days." At the completion of His creation, God announces that it is very good.

Creation Day 7 (Genesis 2:1–3)

God rests. This in no way indicates He was weary from His creative efforts; rather, that the creation is complete. God is establishing a pattern of one day in seven to rest, keeping this day will be a distinguishing trait of God’s chosen people, Israel (Exodus 20:8–11).

Many Christians interpret these "days" as literal, 24-hour periods, certain interpretations of these "days" suggest they were indeterminate periods of time. Regardless, the events and accomplishments of each "day" are the same.

In many languages, the names of the seven days of the week are derived from the the seven classical planets described and observed by ancient Babylonian astronomy (those visible with the naked eye), named after planetary astrological deities, a system introduced by the Sumerians, the earliest of all calendars, dating back to around 3100 B.C. The Sumerian calendar consisted of 12 lunar months, each with 29 or 30 days, used for religious purposes only unlike the everyday use of the Egyptian calendar, adopted from the Babylonians and remaining in use until the Roman Empire adopted the system during Late Antiquity.

The Julian calendar was adopted by Julius Caesar in 46 BC, consisting of 12 lunar months, each with 29 or 30 days. By the Julian reckoning, the solar year comprised 365 1/4 days, and the intercalation of a "leap day" every four years to maintain correspondence between the calendar and the seasons. It became the official calendar of the Roman Church, adopted by Emperor Constantine ca AD 325.

The Julian calendar was replaced by the Gregorian in 1582, proclaimed by Pope Gregory XIII to be a reform, modification and replacement calendar. The beginning of the legal new year was moved from March 25 to January 1, and the Gregorian Calendar utilized a new different formula to calculate leap years. It was immediately adopted in Italy, Poland, Portugal and Spain, and is still regarded as one of the most accurate calendars in use throughout the world.

The Gregorian Calendar is a solar calendar, based on the movement of the Earth around the sun. One year in the Gregorian Calendar has 365.2425 days, divided into 12 months, each divided into 52 weeks, with 7 days each. Seven of the 12 months have 31 days (January, March, May, July, August, October, and December .

The original goal of the Gregorian calendar was to change the date of Easter. In 1582, when Pope Gregory XIII introduced his Gregorian calendar, Europe still adhered to the Julian calendar, 1st implemented by Julius Caesar in 46 B.C. The Roman emperor’s system miscalculated the length of the solar year by 11 minutes, causing the calendar to fall out of sync with the seasons. This meant that Easter, traditionally observed on March 21, fell further away from the spring equinox each passing year.

The Julian calendar added an extra day in February every four years. Aloysus Lilius, the Italian scientist who developed the new system Pope Gregory unveiled in 1582, realized that the addition of these days made the calendar slightly long. He devised a variation that adds leap days in years divisible by four, unless the year is also divisible by 100; If the year is also divisible by 400, a leap day is still added resolving the lag created.

Despite Lilius’ ingenious syncing the calendar with the seasons, his system is still off by 26 seconds, resulting in a discrepancy of several hours accumulated since 1582. By the year 4909, the Gregorian calendar will be a full day ahead of the solar year.

Though Pope Gregory’s papal bull had no power beyond the Catholic Church, Catholic countries swiftly adopted the new system for their civil affairs. European Protestants, weary of its ties to Rome, were apprehensive lest it was an attempt to squelch their movement. Not until 1700 did Protestant Germany switch over, and England not until 1752. Across the Atlantic, Benjamin Franklin welcomed the change, writing, "It is pleasant for an old man to be able to go to bed on September 2, and remain there until September 14."

Orthodox countries clung tenaciously to the Julian calendar until much later, and their national churches to this day have refrained embracing Gregory’s reforms.

Between the 1st and 3rd centuries AD, the Roman Empire had gradually replaced the eight-day Roman nundinal cycle with the seven-day week. The earliest historical evidence for this is a Pompeiian graffito referring to 6 February (ante diem viii idus Februarias) of the year AD 60 as dies solis ("Sunday"). Another early witness is a reference to a lost treatise by Plutarch, written ca AD 100, addressing "Why are the days named after the planets reckoned in a different order from the 'actual' order?" The treatise is lost, but the answer to the question is known; The Ptolemaic system of planetary spheres asserts that the order of the heavenly bodies, from the farthest to the closest to the Earth, objectively, are ordered from slowest to fastest moving as they appear in the night sky.

The days were named after corresponding deities of the regional culture in other languages, beginning either with Sunday or with Monday. The seven-day week was adopted in early Christianity from the Hebrew calendar, and gradually replaced the Roman nundinal cycle as the new religion spread. Sunday remained the first day of the week, as the Lord's Day, while the Jewish Sabbath remained the seventh. Emperor Constantine adopted the seven-day week for official use in AD 321, making the Day of the Sun (dies Solis) a legal holiday.

The days were named after the seven planets of pagan Astrology, in the order:

Sunday "Sun's day," the Sun is personified as Sunna/Sól.
Monday "Moon's day," the Moon is personified as Máni.
Tuesda "Tiw's day," Tiw (Norse Týr), "Day of Mars" (the Roman god of war).
Wednesday "Day of Woden/Ódinn," "Day of Mercury," Magic - Knowledge Deities.
Thursday "Thor - Thunder" "Day of Jupiter" (the Roman god of thunder).
Friday "the goddess Fríge's star' "Day of Venus."
Saturday (originally Shabbat=Seventh), "Day of Saturn," (the Roman god Saturn/Titan Cronus).

The Hebrew months were originally only numbered, but over time names were given to them. For instance, Abib, the first month of spring, means "green [ears of barley]" (this month is also called Nisan, meaning "their flight [out of Egypt]"). Later, the Jews borrowed Babylonian names for many of their months, some of which (e.g., Tammuz) refer to pagan deities.

Month From the Latin

January - Januarius, in honor of the Roman god Janus.
February - Februarius, in honor of the Roman festival of general expiation and purification.
March - Martius, in honor of the Roman god Mars.
April - Aprilis, which was derived from aperio, a Latin verb meaning to open. The month is so called because it is the month when the earth opens to produce new fruits.
May - Maius, in honor of the Greek goddess Maia.
June - Junius, in honor of the Roman goddess Juno.
July - Julius, in honor of Roman emperor Julius Caesar.
August - Augustus, in honor of Roman emperor Augustus Caesar.

The rest of the months — September, October, November, December — are derived from the Latin words for the numerals 7, 8, 9, and 10. They were the 7th, 8th, 9th, and 10th months of the old Roman calendar before July and August were inserted ahead of them.