Leap years keep our calendar and seasons in sync with the Earth’s orbit around the Sun
The Earth takes about 365.25 days to complete a full orbit around the Sun. If we kept it at exactly 365 days every year, our seasons would slowly drift off course because of the extra 6 hours it takes to orbit. For instance, without leap years, the summer solstice could eventually occur in December in North America. So, every four years, we add an extra day to make up for those missing quarter days.