The year 2020 is a leap year. This means that it has 366 days instead of the usual 365 days. But why does this happen? If you are wondering why there is a leap day or why there is a leap year, here are the details about the mystery behind this extra day, who added it and when, and how to calculate leap years.

The leap year exists because one extra day is added to the calendar every four years to make it synchronize with the actual solar year. The Earth's orbit around the sun actually takes around 365.25 days - there is an extra one-fourth day.

Thus, to align the calendar year and the solar year, we decided to add one day every four years. This is because 0.25 days taken four times (for four years) would add up to a full day.

In a year which is not a leap year, that extra one-fourth day is not taken into consideration.

What happens if there is no leap year?
In case there is no leap year and every calendar year has 365 days, it will go slower than the actual solar year.

As the years go by, the gap will become wider until we would not be able to estimate our seasons and the time of the year anymore.

If there is no correction, the calendar year will be off by about one day after four years. After 100 years, it will be off by 25 days. Gradually, the gap will widen so much that February would turn into a harsh summer month.

History of leap year
The concept of leap years can be traced all the way back to the Egyptians who had devised a solar calendar featuring 365 days with the insertion of a full 'leap month' when the astronomers observed the right conditions in the stars.

This arbitrary method of the ancients was carried forward till Julius Caesar got tired of it. Instead of depending on the stars, Julius Caesar simply added an extra day every four years to the Julian Calendar back in 46 BC on the advice of an Alexandrian astronomer named Sosigenes.

The Julian Calendar which was the first to introduce a fixed leap day to manage the mismatch between the length of the actual year and the calendar year came into effect from January 1, 45 BC.

In 1582, another mistake came to be noticed. This is because even though the Julian Calendar considered every solar year to be of 365.25 days, an actual solar year was 365.2422 days. So, taking one year as 365.25 days meant there was an extra 11 minutes.

Tired of the calendar devised by Julius Caeser, Pope Gregory XIII decided to create his own calendar and cleverly introduced the concept of "skipped leap years".

The method to calculate and place leap years in the Gregorian calendar was devised by a German mathematician and astronomer Christopher Clavius (1538-1612). He assisted Pope Gregory XIII in creating the Gregorian calendar.

This Gregorian calendar was accepted as the international standard for many countries across the world.

How often is a leap year and what are 'skipped leap years'?
Does a leap year take place every four years? Most of us think that a leap year takes place every four years and that the easiest way to calculate leap years is to check if a year is divisible by 4. But that is not true.

If we notice the fine calculations, we will see that a solar year occurs over a period of 365.2422 days, which is less than 365.25 days. So even if we are adding one extra day every four years, it is overshooting the solar year by a slight margin.

This is why we have a concept of skipped leap years. To fix this tiny misalignment and keep the calendar steady for thousands of years, we drop a leap year every few years.

The way Pope Gregory XIII fine-tuned the calendar year to this extent so long back is impressive indeed.

The Gregorian Calendar has the last major change in the western calendar. But we would need another correction soon again since the Gregorian calendar is slightly off as well - but only once every 3030 years. So we still have time to figure a way out!

Why is there a leap day in February?
The only reason February was chosen to house this extra day or leap day was that all the other months were already rather "full". But we should note that February didn't always have 28 days.

As per history.com, February used to have 30 days too but Caesar Augustus, the nephew of Julius Caesar, was not pleased with the fact that the month named after his uncle - July - had 31 days but August didn't.

Since August was a favorite day of August as he had won several battles this month, Augustus took two days from February and added them to August.

How to calculate leap years?
It is very much possible to calculate leap years and know in advance which years will have a leap day.

We already know that years which are divisible by 4 are leap years, barring exceptions -- the Gregorian calendar stated that no leap days would be added in the years ending in "00" unless that year was also divisible by 400.

For example, the year 2000 was a leap year (it is divisible evenly by both 100 and 400), but the years 1800 and 1900 were not leap years (they are both divisible by 100 but not divisible by 400). These are examples of skipped leap years.

So once you know how to calculate leap years, you can figure out the leap years after 2020:

2024 2028 2032 2036 2040 2044 2048 2052 2056 2060 2064 2068 2072 2076 2080 2084 2088 2092 2096 2104 2108 2112 2116 2120 2124 2128 2132 2136 2140 2144 2148 2152 and so it continues.

The next skipped leap year will be 2100.

List of leap years since 1600
As per the Gregorian calendar, here is a list of leap years since 1600:

1600 1604 1608 1612 1616 1620 1624 1628 1632 1636 1640 1644 1648 1652 1656 1660 1664 1668 1672 1676 1680 1684 1688 1692 1696 1704 1708 1712 1716 1720 1724 1728 1732 1736 1740 1744 1748 1752 1756 1760 1764 1768 1772 1776 1780 1784 1788 1792 1796 1804 1808 1812 1816 1820 1824 1828 1832 1836 1840 1844 1848 1852 1856 1860 1864 1868 1872 1876 1880 1884 1888 1892 1896 1904 1908 1912 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016 2020

What if you have a birthday on February 29?
There is a one in 1461 chance of being born on a leap day and people born on this rare day are called 'Leaplings'.

The city of Anthony in Texas, US, has an elaborate celebration with birthday parties and parades for these 'Leaplings'.