Every four years we add February 29th to the calendar. This year, 2016, is one of those years. We call these leap years. Why? Earth's orbit around the Sun takes 365.242199 days. We have 365 days in the calendar, so if adjustments are not made, the seasons will shift dates on the calendar. Summer in July will eventually happen in December. To keep the calendar consistent with the seasons we add an extra day every 4 years.
The length of the year, however, is NOT 365.25 days, so adding a day every 4 years is not enough. In addition every year divisible by 100 (1900, 2100, for example) are NOT leap years. If the year is also divisible by 400, it IS a leap year. So although the year 2000 is divisible by 100, it is also divisible by 400 and is therefore a leap year. Confused? LOL! Blame the Earth and the Sun for not aligning properly to have an orbit of exactly 365 days!