The last leap day was on 29 February 2020 (Saturday) while the next after 2024 is on 29 February 2028 (Tuesday).
Why add a leap day? Leap days are needed to keep our calendar in alignment with the Earth's revolutions around the Sun.
Besides that, It takes the Earth approximately 365.242189 days—or 365 days, 5 hours, 48 minutes, and 45 seconds—to circle once around the Sun. This is called a tropical year.
Without an extra, or intercalary day on February 29 nearly every four years, we would lose almost six hours every year. After only 100 years, a calendar without leap years would be off by approximately 24 days. Seasonal days such as the vernal equinox or the winter solstice would, therefore, shift in relation to the months in the calendar. For example, in 100 years, the Northern Hemisphere's autumnal equinox which falls in late September would fall in late August and in a few centuries, August would become a spring month.
