The History of The Calendar

It is unclear who first came up with the idea of a 365-day year, but it is believed to have been used by the ancient Egyptians as early as 4236 BCE.
It was later adopted and perfected by the Romans, who named each month and designated specific days for religious observances and government meetings.
The concept of leap years—adding an extra day every four years to account for the extra quarter of a day in the solar orbit—was also introduced by the Romans.