Everyone shifted to the Gregorian calendar because it is more accurate than Julian calendar.
A normal Julian year is 365.25 days and its an error of 11 minutes when compared to the solar year. It takes the earth 365.2422 days to complete one solar orbit This error in the Julian calendar cause an extra day every 128 years of the Julian calendar.
Gregorian calendar corrects this error by removing 1 day in years that are divisible by 100 but not divisible by 400. Example 1700, 1800, 1900 and 2100 are not leap years.
Many protestant countries believe the Gregorian is a plot by the catholic because Julian Carl was making some errors and Easter was gradually occurring later in the Spring over the years, if something was not done, Easter could occur in Summer or Fall.
Catholic countries like Italy, Spain and Portugal adopted the Gregorian calendar quickly. Protestants countries saw this new calender by the catholic church as a way to silence their movement.
To-do lists cannot be measured in time. Most people use them to ensure that certain tasks are carried out, and these are crossed off as they are achieved. There is no measurement of time involved in a to-do list. In theory, if all we had was this there would be no way of ensuring a certain task was carried out on a certain day, because how would the days be predicted or measured without a calendar?
How could we arrange with another person when to meet and manage to get there are the same time? How would we plan our year. the to do list might take a week or a year and might be very different from other peoples lists whose actions corresponded to each other.
Gregorian calendar is used by most countries. There are countries that do not use this calendar. Some of the countries that do not use Gregorian calendar are: Afghanistan, Iran, Ethiopia and Nepal.
Afghanistan and Iran use the Solar Hijri Calendar, Ethiopia uses the Ethiopian calendar, Nepal uses Vikram Samvat.
Some countries use Gregorian calendar with other calendars this includes countries like: Bangladesh - use Bangla calendar, India - uses Indian National calendar and Israel - use Hebrew calendar.
The Germans and the Romans invented the day system. The names used to describe the day of the week was taken from Hellenistic astrology and were named after the gods in folklore. While the Romans first introduced the system, the Germans substituted the Germanic deities for the Roman ones except for Saturday. This was called interpretatio germanica.
The timeline when this happened is not really known but historians estimate that it happened sometime after 200 AD but before the 6th and 7th century. The specific days of the week got their names from German languages, particularly West German. Then the names were changed to be named after different deities. Some cultures have the week beginning on Sunday and others have the beginning of the week starting on Monday.
The Egyptians were the first to move from measuring time by the moon, to by the sun, which made eleven more days in the year. It was the Romans developed the calendar. Julius Caesar, in fact, in 45 BC.
His calendar didn't stay in use because we all changed to the Gregorian calendar in 1582 thanks to Pope Gregory XIII It was a refinement of Caesar's calendar
There are actually seven or more calendars around the world and their study is very interesting. Although the Gregorian calendar is the one most widely in use, certainly in the western world, there are also its predecessor, the Julian Calendar, the Hebrew Calendar., the Hijri Calendar, the Iranian Muslim Calendar, the Buddhist Calendar, the Vikram Samvat, Shaka Samvat, and Kali Yuga calendar, the Japanese Calendar and Chinese Calendar. the Zodiac calendar is one that many find fascinating. However, the Gregorian is the one that overcame the solstice and equinox problem with measuring time passing.
The calendar we follow is the Gregorian calendar, if by 'we' you mean the western world. The Gregorian calendar is a refined and improved version of the earlier Julian calendar, desgined by the Romans, specifically, julius Caesar.
He began his calendar with January 1st, naming the happy summer month of July after himself. The current Gregorian calendar is the one most widely used, and associated with the west.
If by 'people' you mean all of us living in the western world then it is clear that the Gregorian calendar is the most widely used. To calculate 'mostly used' you would have to use the populations of the various countries that depend on each calendar. 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.
Actually most European calendars start the week on Monday. Slavic languages number Monday as the first day too. It is North American calenders that start on a Sunday. This is because it is the first day of the week in Jewish culture and early Christian times. So, the answer if you want to buy a calendar starting on a Monday is to buy a European one.