why do we have a leap year every 4 years

Leap years exist to keep our calendar aligned with the Earth's orbit around the Sun. The solar year is about 365.2422 days, not exactly 365 days, so if no adjustment were made, the calendar would drift with the seasons over time. By adding an extra day every four years, we compensate for the roughly 0.2422 days per year, which accumulates to about one additional day every four years, keeping spring, summer, autumn, and winter in their roughly expected positions on the calendar. However, to maintain even greater accuracy, that four-year rule is refined: century years are not leap years unless they are divisible by 400. This means years like 1600 and 2000 are leap years, but 1700, 1800, and 1900 are not, while 2000 was a leap year and 2400 will be as well. This adjustment further reduces drift over longer timescales. If you’d like, I can tailor this explanation to a specific audience (students, colleagues, or a general audience) or add a simple timeline illustrating how the accumulated fractional days map to leap years over centuries.