How do humans decide how long a year should be?

Humans have defined the length of a year based on the Earth's orbit around the Sun, which takes approximately 365.25 days. To account for the extra quarter day, we add a leap day every four years, resulting in a leap year of 366 days. This system is formalized in the Gregorian calendar, which is widely used today. The choice of a year length is fundamentally tied to astronomical observations and the need for a consistent framework for timekeeping.