Unlike animals, human beings need structure in their lives. They need to know how long events occur, to predict when things will happen and to make arrangements with other humans that are time based. For all of this they need to measure time and communicate these measurements so that their lives are more ordered.
They also have an emotional need to celebrate or commemorate key events, such as birthdays, and a measurement of time that is reliable and widely understood enables this. A calendar is the result, and calendars have been used for thousands of years, probably more.
Gregorian calendar was introduced by Pope Gregory XIII which explains how the name came about.
Julian calendar has 2 types of years: Normal year and Leap year.
The normal year has 365.25 days and the leap year has 366. Normal year is actually 11 minutes less.
Gregorian calendar makes up for the 11 minutes missing in normal Julian calendar by making years divisible by 100 but not divisible by 400 to be a leap year. For example, 1700, 1800, 1900 and 2100 are not leap year.
Ethiopia does not follow Gregorian calendar because the country uses it's own calendar - Ethiopian calendar. The calendar is used in Eritrea and Ethiopia, it is used as the liturgical year for Christians in Eritrea. The calendar begins on August 29 or August 30.
The Ethiopian calendar is 7 - 8 years behind the Gregorian calendar. The calendar was derived from Egyptian calendar. The calendar is similar to Julian calendar, it adds a leap year every 4 years with out exception.
The idea of dividing time into periods such as years, months, and weeks developed later, but the concept of a calendar really began in the Bronze Age. There is evidence that people then were marking time, trying to manage and predict events by using the natural rhythm of our days.
The Persians certainly showed this desire to have close awareness of the passage of time but the calendar as we know it began with Julius Caesar, the Julian calendar, later to be replaced by the Gregorian calendar we use today.
I can only speak for the western world, but I think it would be sensible to assume that events and dates of these events decided when festivals were held, that is, to celebrate or remember key happenings such as the date someone famous died or was born or won some battle. The exception is religious festivals, and it seems that these tend to coincide with weather changes of the year.
In past times, the coming of Spring after the misery of winter was a time to celebrate new growth. This was the time of year when sheep lambed, as well as buds appearing on the tree. It was appropriate for the rising of Christ to be aligned with this, and Easter became an established festival. This is an example, Christmas, harvest being others.
Calendars help us keep track of time passing and of structuring our future, making plans and communicating these to others. Without calendars, we would be making scribbled notes and hoping this would liaise with the notes others were making.
Calendars help us to stay organized, to remember important dates including those only relevant to our family like birthdays. It is difficult to maintain any kind of efficient schedule without recourse to calendars. The whole world has found this, for some form of calendar is found wherever people have a civilized society.
As far as I know, the calendar most used, the Gregorian calendar, is the most accurate. It takes the number of days the Earth takes to circulate the sun and with the odd bit left over, a leap year every fourth year, brings the movement of time into a neat calculation. The Gregorian is the calendar used in the international standard for Representation of dates and times.
Because it calculates the year from the time it takes the Earth to circulate the sun, it is the most accurate way of calculating the passing of time, making sense of the term A Year. the Gregorian calendar aligns with the movement of the Earth around the Sun and also corresponds to changes in seasons at different latitudes.
I don't know about 'most'. This depends on where you live. In the United States calendars begin on a Sunday, but not all over the world. Slavic languages have Monday as the first day and Monday is used all over Europe as the first day in the week.
Sunday was named as first day of the week in Jewish culture and Hebrew, Arabic, Greek, and Portuguese names for the days follow this thinking.
Great Britain shifted to the Gregorian calendar in 1752. The year 1751 was a short year (282 days) running from 25 March to 31 December. 1752 was also short of 11 days.
Spain, Italy and France shifted to the Gregorian calendar in 1582, Greece shifted in 1922, Russia shifted to Gregorian calendar in 1918. Many countries didn't shift to Gregorian calendar until recent time.
The main reason why Gregorian calendar was implemented was because Pope Gregory XIII wanted to change the Easter date to be in sync with the time it was celebrated by the early church. This happened in 1582.
The Roman Emperor's system miscalculated the length of the year by 11 minutes which made the calendar fall out of sync with the season.