A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. It takes approximately 365.25 days for Earth to orbit the Sun, which is why we usually round the days in a calendar year to 365. However, because we’ve subtracted approximately 6 hours — or ¼ of a day — from 2017, 2018 and 2019, we have to make up that time in 2020. That’s why we have a leap day. The rules for leap years in the Gregorian calendar are:

• The year must be evenly divisible by 4.
• If the year can also be evenly divided by 100, it is not a leap year, unless...
• The year is also evenly divisible by 400. Then it is a leap year.

For example, the years 1700, 1800, and 1900 were not leap years, but the years 1600 and 2000 were. The intercalary day that usually occurs every four years is called the leap day and is created by adding an extra day to February. This day is added to the calendar in leap years as a corrective measure because the Earth does not orbit the Sun in precisely 365 days.