How did our current calendar come into being? Why do we have a calendar at all? Well at calendar.com they provided some insight brought to you below:

"Wherever you live, whatever language you speak, and however you fill your day — that day will be the same as everyone else’s, in one significant way. The sun will rise in the morning and set in the evening. The sun tends to be quite predictable and follows a regular anticipated cycle. We can set our watches by the sun — but a millennia ago, the sun bore a child — the calendar. Here are some interesting facts about the history of the Calendar.

The Nature of the Sun Brought with it Enough Calendar History to Produce a Calendar.

Very few things in this world can be depended on — but the sun is loyal, steady, and true. Its cycle is trustworthy year after year — making its emulation by all people through ages understandable.

At the same time each month, the shape of the moon will change. It will start as a crescent that fills the night sky, then shrinks; a process that takes about thirty sunsets and sunrises. The stars, too, will move across the sky, returning to their original positions after about 365 of those sunrises and sunsets.

Humans have noticed the night patterns for as long as they’ve had backs straight enough to stand and look to the sky. And they’ve tried to predict and measure those movements too, and for good reason. By counting days and the passage of the moon, they could predict changes in the weather.

These ancient people could tell when winter was approaching by when the days would grow longer or shorter. They would know when to plant crops; when to look for particular animals; when their own animals were likely to give birth, and when to give thanks to the gods.

Today, our history tells us to count those days to plan meetings, book vacations, plan events and a host of other things on our Calendars.
Our history depends entirely on the use of a calendar to organize our days, now, in our time. In this guide, we’re going to look at how the calendar has developed and how we use it today.

Calendars in Ancient Times

In 2013, British archeologists announced the discovery of what they claim as the world’s oldest calendar. The site at Warren Field in Scotland consists of twelve pits aligned with the southeast horizon. They pointed towards a hill associated with the sunrise on the midwinter solstice. Archeologists believe that hunter-gatherers used the pits to check the height and stage of the moon in order to track time in relation to the sun and the changing seasons.

The calendar in Scotland is about 10,000 years old, which makes the Warren Field in Scotland about twice as old as Stonehenge (discovered in 1978). People are more familiar with Stonehenge sight, an ancient stone circle in the south of England, which also aligns with the solstices.

The challenge with interpreting these sights, though, is that Neolithic people created and built the sights at a time when there were no written records. Archeologists have looked at the shape and alignment of the stones and the contents of nearby burial sights to figure out what other practices were conducted here, and what other secrets the sights may hold.

Stonehenge is more likely to have been a location for performing rituals at specific moments of the year than a way to keep track of time — although the structure is capable of being a calendar, also revealing times of the equinoxes and solstices — (which are not precisely the same thing). Recent findings show the Stonehenge sight was believed to hold curative, healing powers. Hunters might have used the Warren Field (Scotland) not only to give them “times of the year to plant or harvest,” but possibly to tell hunters when they could expect to start looking for particular kinds of migrating animals.

Evidence for some abilities needed to wait for the start of civilization and the first written calendars.

While early man might have used both sights used to mark time, some people say it’s unlikely that they used them to keep track of time permanently. The sights show that Neolithic peoples had a concrete concept of time passing and knew that cycles were predictable over time. Some sights indicate an ability to measure the passing of weeks or months.

The History of the Babylonian Calendar

The first cities were formed between the Tigris and Euphrates rivers that originated in the Taurus mountains of southeastern Turkey. The headwaters diverge and run south through Syria and Iraq, and several tributaries are added from Iran before flowing into the Persian Gulf.

Ur, which was founded around 3,800 BC, would once have been a coastal city. Changes to the landscape now place it more than 200 kilometers from the sea; but at one point the Ur III empire would have stretched up through much of modern Iraq, incorporating a number of smaller cities.

Clay tablets marked by cuneiform writing indicate that before Ur incorporated them, those cities would have had their own calendars with their own names for the months of the year. Nippur, for example, had months called “du6-ku33,” or “Shiny Mound,” and “kin-dinanna,” or “Work of Inanna.”

The city of Umma had months that translate as “Harvest,” “Barley is at the quay” and “Firstfruit (offerings).” Each of the cities had a month called, “Extra,” that allowed them to reset the calendar in the same way as a leap year.

The conquests of King Shulgi, who ruled in the 21st century BC, united those calendars into the Umma calendar — and that calendar formed the basis of the Babylonian calendar. The Umma calendar, too, had twelve months and a thirteenth month every four years.

The calendar starts in the spring, around March or April in the Gregorian calendar, with Araḫ Nisānu, the “Month of the Sanctuary.” That’s followed by the “Month of the Bull,” which corresponds with the zodiac sign Taurus. The seventh month is the “Month of the Beginning,” and it begins the second half of the year — and is followed by the “Month of Laying Foundations.”

Babylonian weeks would not have been unfamiliar. Each seventh day was a rest day on which officials were prohibited from engaging in certain activities. For the Babylonian calendar — these same activities couldn’t be done on the 28th day of each month, either. On each rest day, Babylonians made offerings to a different god. Perhaps the strangest aspect of a Babylonian month would have been the length of the last week. Each week lasted seven days, but during the lunar cycle, the month, lasting 29 or 30 days, made it so the last week of each month lasted eight or nine days.

The History of the Egyptian Calendar

The Babylonian Empire lasted from around 1896 BC to 539 BC, reaching its peak during the reign of King Hammurabi (1792 BC to 1750 BC). At the same time that Babylonians were looking forward to the lengthy last weekend of the month, the Egyptian empire was growing in the west.

Scholars dispute the existence of early Egyptian calendars based on the rise of Sirius or the presence of a year lasting 360 days. But it’s clear that as early as 3,000 BC, Egyptians were interested in the yearly cycle. What interested them most was the annual flooding of the Nile. Each year between May and August according to the Gregorian calendar, the monsoon brings heavy rains to the Ethiopian highlands south of Egypt. The waters flow into the Nile, causing the river to flood its banks.

That flooding determined the size of the harvest. A system of dams and dikes drove the floodwater into fields to saturate the soil. The water that collected in the fields had to be sufficient to grow the crops through the dry season. A low flood meant a poor harvest.

But the floods also determined the pattern of the year. Egyptians divided their calendar into three seasons. The Flood Season lasted from around June to September and was when the Nile flooded and the waters inundated the fields. “Emergence” lasted from around October to January. Finally, the Low Water or harvest season took place between February and May. During the early dynasties of Egyptian history, the months within those seasons had numbers —“First Month of the Flood,” “Second Month of The Flood,” and so on. By the Middle Kingdom, however, the months had picked up names which have largely survived through the New Kingdom and Greek calendars to the current Coptic calendar.

The Calendar According to the Nile

The three periods determined by the rise and fall of the Nile formed the year, and they occurred regularly. They were as crucial as migrating birds were to the hunter-gatherers of the Scottish Isles — yet, no more predictable. While the rise and fall of the sun and the moon or the turning of the stars follow a definite pattern, the coming of the monsoon, like the flight of storks, depends on weather systems that vary from year to year.

While the seasonal calendar allowed Egyptian farmers to predict when they should open their dams and plant their seeds. The seasonal calendar was decidedly less helpful at predicting and marking other events that took place throughout the year.

By the time of the Old Kingdom, the period when the pyramids were built, Egypt had also instituted a civil calendar. The civil calendar is likely to have been based on the movement of Sirius, a star which re-appeared in the sky at about the same time as the Nile would start to flood.

The civil year was made up of twelve months of 30 days, and an additional month of five days, creating a year of 365 days. The lack of a leap year meant that the movement of the stars gradually fell out of sync with the names of the month. As the appearance of Sirius fell back through the calendar, the Egyptians created a Sothic Cycle. Every 1,461 Egyptian civil years, Sirius would return to its place in the calendar.

The Roman Calendar

By the time of the establishment of the Roman Empire, we had several millennia of experimentation completed with various calendrical systems and with multiple different ways of marking time. We had stone circles and stone markings. We had the lunar calendars and combinations of solar and lunar calendars.

We still weren’t getting it right. That the amount of time the Earth took to revolve around the sun couldn’t be counted in whole days. When a civilization couldn’t count an entire day, it meant that calendars in different places and times around the world would regularly fall out of sync with the seasons. If a calendar is out of sync with the seasons — it’s out of sync with the stars and the movement of the moon.

The earliest Roman calendars were little better than most (and look at that tile work!). These calendars, too, started as lunar calendars, tracking the development of the moon over 29.5 days. With the early Roman calendars, they only lost ten or eleven days a year. At the same time, early Rome also had a nundinal cycle derived from the Etruscans. The nundinal cycle was an eight-day week, ending with a market or a festival. Farmers would head to the city to buy and sell goods. Children had no classes on that day, and slave-owners warned their property not to enjoy themselves too much.

A year in early Rome is believed to have been made up of 38 such nundinal cycles, divided into ten months of 30 or 31 days. How the Romans dealt with the remaining days is unclear. Some scholars have claimed that the Romans disregarded them while others suggest that the early Romans practiced intercalation, inserting extra days into the calendar to fill the gap and make sure that the calendar didn’t fall out of sync with the seasons.

Beware the Ides; and the Kalends; and the Nones
The end of the Roman kingdom and the growth of the Roman Republic led to another revision of the calendrical system. Now the Romans were influenced by Greek calendars which divided the year into twelve lunar months, alternating between 29 and 30 days. The Romans, however, gave the third, fifth, seventh, and tenth months of 31 days each. Every other month was 29 days, except February which had 28 days and 29 in each leap year.

Each month, too, was carefully divided. The Romans called the first of the month “kalends,” the origin of the English word “calendar.” They called the day before the middle of the month the “ides,” and the eight days before the ides (or nine days counting inclusively) they called “nones.” Those moments are likely to reflect the calendar’s lunar origins, and mark the sighting of the crescent moon, the quarter moon, and the full moon.

We can begin to see and understand a bit of information about the foundation of the calendar that we use today. In particular, we note the use of an intercalary month in February to keep the months aligned with the seasons. But the Roman calendar did have one interesting difference.

After the establishment of the Roman Republic, control over the intercalation passed to the high priests. By adjusting the number of days in February, they were able to lengthen or shorten the term of office of the consuls they supported. It was as though a political party could determine the length of a year and make the year longer when they were in office. The priests could gerrymander the calendar.

The History of the Julian Calendar

In 48 BC, Julius Caesar proposed a reform to the Roman calendar. The high priests’ willingness to adjust the length of the year in accordance with the rule of their political allies or to skip intercalation if it suited them, meant that the calendar drifted out of alignment with the year. The Second Punic War against Carthage and the Civil Wars, in particular, meant that few people outside Rome knew the current date. Between 63 BC and 46 BC, the calendar had only five intercalary months instead of eight, and none between 51 BC and 46 BC. Historians have called these years the “years of confusion.” Julius Caesar had spent time in Egypt, knew what day it was, and wanted a more conventional way of maintaining the calendar.

After returning from the African campaign in 46 BC, Caesar added two intercalary months between November and December, increasing that year by 67 days. The year had already been increased from 355 to 378 days, so in 46 BC the calendar was now 445 days long.

The reform then added ten days to every year. Two days were added to January, Sextilis (which is now August) and December. Another day was added to April, June, September, and November. February continued to be 28 days. The new calendar removed the previous intercalary month, replacing it with a new leap day placed before the kalends of March. Romans continued to mark kalends, ides, and nones, but the pattern of the calendar that would come to be used by much of the modern world had been formed."

There is more, but that will have to be tomorrow.

Reference: https://www.calendar.com/history-of-the-calendar/