What are leap years?
Many people don't know how leap years work and why they exist. To understand this concept, one must be aware that one common (non-leap) year on the Gregorian calendar is NOT 365 days. Rather, it is 365.2422… and so we must account for that extra part of a day.
Over four years, the portion unaccounted for, almost a quarter of a standard 24-hour day, adds up to just short of one full day (approximately 97% of a day in length). Thus, anyone who follows the Gregorian calendar tacks this extra "day" to the end of the shortest month of the year, February. The way this works out, leap years are divisible by 4 unless they fit another scenario: divisible by 100 but not by 400. This extra addition to the rule has to do with that extra ~3% of the day that we fabricate when we add a whole (100%) day. This extra addition allows the years 1600 and 2000 to be leap years, and will also allow the years 2400, 2800, 3200, and so on, to be as well.
Why leap years?
In reality, the solstices and equinoxes happen the same time every year, it's just a matter of how it compares with our calendar (and also the delay of the finite speed of light). If we didn't account for the extra 0.2422 days that builds up overtime, then every four years, the solstices and equinoxes would shift one day off from the previous year's calendar day. This means that the seasons would change months over many centuries and, depending where you're from, snow would fall in every month at some point in time, although not in the same year.
Who came up with this stuff?
Overtime horologists (i.e., clock and time scientists) studied variations in time and date as compared to the sun's location and the seasons. Eventually, the conventional leap day came to be in order to fix, or rather lessen, these variations to a manageable size.