What is a Leap Year?
Why Does This Happen?
The calendar year actually follows the schedule of the Earth's orbit around the sun which takes about 365 and 1/4 days. The extra hours add which is what causes the extra day every four years so the calendar year matches the solar year again.
When Are Leap Years and How Can You Tell?
Leap years happen usually every four years, but sometimes a leap year has to be skipped to keep the calendar in sync. To tell whether or not a year is a leap year, there is a very easy system. Years that don't end in 00 are leap years if you can divide them by 4. Century years, years that do end in 00, are leap years if you can divide them by 400.
Since experts added the extra day to the calendar, it set the seasons in a more concrete pattern. Without the addition of this day the seasons would drift around the calendar. For example, over a span of 300 years, July would move from summer to spring.
VeggieTales: Larry's Leapin' Leap Year