I think the main reason was to know when the times for planting and harvesting were approaching. Another big reason was knowing when to celebrate religious festivals.
Calenders were invented to keep track of when it would get cold and when it was a good time to plant crops at first
The calendar was technically first invented by the Ancient Egyptians.
The Gregorian calendar was an adaptation of a calendar proposed by Aloysius Lilius in 1582. However, the calendar is named after Pope Gregory XIII who introduced this calendar by a papal bull. It was a reform to the Julian calendar.
Properly speaking, there is no "English" calendar system. You probably mean the predominant Western Calendar (also called the Christian Calendar). This calendar system is best called the "Gregorian Calendar", after Pope Gregory XIII who introduced the calendar system in 1582.
Historians do not know who or when mankind invented the first calendar, probably early man when he first carved a notch into a stick or a bone which marked the passing of each full moon. Notched bones used to record moon phases have been found in Africa and Europe dating back to about 20,500 BC. There are many different calendars invented by different cultures to suit their needs. The mayans created a calender, but I believe the calendar we use is based off of the Romans. Some also believe that the Sumerians invented the 12-month calendar.
Julius Caesar is credited with changing the Roman calendar from 355 days to 365 days and for adding leap year days every few years (and for changing the name of Quintilis to July in honor of himself).
George Washington invented the lunar calendar
The name of the person that invented the first calendar is not known. The first calendar was invented in Scotland.
China
It is not known who invented the lunar calendar. However, the ancient Chinese are among the first people who begin using a lunar calendar.
The Egyptian
Rome
The calendar was technically first invented by the Ancient Egyptians.
The Romans invented it in 46 ad.
I t was invented around 4231 B.C.
It is not know precisely who or where the first 365 day calendar was invented or used. Many cultures on many continents used and invented it independently.
In Calendaria, Babalonia.
173 a.c.