Why is a year 365.5 days long?

A year is approximately 365.5 days long due to the Earth's orbit around the Sun, which takes about 365.25 days. To account for the extra quarter day, we add an additional day every four years, resulting in a leap year with 366 days. This adjustment keeps our calendar aligned with the Earth's position in relation to the Sun, ensuring that seasons remain consistent over time. The average length of a year in the Gregorian calendar is thus effectively 365.2425 days.