The year 2020 is a leap year, so an extra day is added to a usually 28-day month of February.
Of course, there is a specific purpose behind this year’s 366-day calendar, it helps keep our calendars aligned with the Earth’s rotation around the sun. The modern calendar contains 365 days, but the actual time it takes for Earth to orbit its star is slightly longer—roughly 365.2421 days.
It might seem negligible, but over decades and centuries that missing quarter of a day per year can add up. So to make sure there is consistency with the true astronomical year, it is necessary to periodically add in an extra day.
The leap year was introduced in the Julian calendar in 46 BC with the creation of the Julian calendar. The emperor included a leap year every four years. But Caesar’s math wasn’t quite right.
Earth completes one orbit in 365 days, 5 hours, 48 minutes and 46 seconds. However, with three years of 365 days and one leap year of 366 days, the average length of a year in the Julian calendar was 365 days and 6 hours. This was longer, though slightly, than the time taken by the Earth rotate around the Sun.
In the 16th century, it was calculated that the calendar years until then had accumulated 10 extra days.
Now, a leap year occurs in every year that is divisible by four but only in century years that are evenly divided by 400.
But even now the calendar is not completely correct! Because it takes 365.2425 days for the Earth to travel around the sun, the solar year is approximately 26 seconds shorter than the Gregorian year at the moment. It will take 3,300 years before this discrepancy becomes a problem.