The solar year (length of time the Earth takes to move around the sun) lasts 365.2422 days, this remaining amount of 0.2422 days makes an enormous difference; this is why we must turn to certain mechanisms to adapt it to the calendar year in such a way that there is almost no difference between them. The mechanism used to adjust that difference is the leap year.
When is leap year?
Every four years is leap year, but it’s not that simple, every hundred years is NOT leap year and every four hundred years IT IS leap year.
What would happen if there were no leap year?
Each year, the solar and the calendar year would both get a little out of phase. In the beginning it wouldn’t be noticed, but a couple of years after, let’s say 60 years, the difference of around 15 days would then be important in such way that equinoxes and solstices would start to occur in dates each time further and further away from the originals. 
If there were no leap years every 4 years, this difference would be of almost a full day; same difference that is recovered in a leap year.
In this animation in the cosmic room, it is feigned that there is no leap year; resulting in a projection that moves substantially away from the reference square. 

We already consider a leap year in this video. Notice that every four years, the projection returns to the reference square.


Is it enough to have one leap year every four years?
It is in fact more than enough, given that when we add one day in 4 years, we increase from 365.00 to 365.25 days, with which we exceed the solar year in 0.0078 days.
What would happen if we lived with this small difference of 0.0078 days?
It wouldn’t happen much in shortterm, in fact The Romans did it that way with the Julian calendar; it’s just that as centuries passed by, they accumulated this difference until they reached 11 days, which they decided to adjust with the Gregorian Reform eliminating the days of difference, going from being on a Tuesday, October 5th 1582 to being the next day on October 15th 1582, eliminating at a stroke these spare days. 
If we watch the following video, we can see how the marks of the sun move a little every 4 years.
Observe carefully since the change is almost imperceptible.


What do we do to reduce that difference of 0.0078 days?
Every 100 years a day is taken off in the following way: every 100 years the year is not a leap year. This way, we subtract 0.01 from our previous calculation, going from 365.25 to 365.24 and remaining only an excess of 0.0022 days with the solar year. (365.2422–365.2400= 0.0022).
What would happen if we live with this little difference of 0.0022 days?
Once again, in shortterm it would be of no importance, but the fact that they made this implementation in 1582 made it possible for the cycle to be completed in the year 2000 in the following way:
Every 400 years, even though it doesn’t match the previous rule, the leap year is incorporated thus raising the calculation of our calendar year 1/400 (0.0025) and reducing the difference to only 0.0003 days. (365.2425365.2422=0.0003).
In this clever and simple way, we match the calendar year with the solar year.







