The equinox is a mystical moment in time: it is one exact second when the sun is directly above the equator, and the day and night are precisely the same length. This happens twice a year, in spring and fall. So when exactly is the spring or vernal equinox? It turns out this is a much more complicated question than it seems at first. The short answer is that it can fall anywhere on the 19th, 20th, or 21st of March. Which day it will be each year has to do with our current 365-day calendar system and how it relates to how long Earth actually takes to orbit the sun. It so happens that this year, it will land on March 19, which hasn't occurred since 1896.
Now let's get into some calendar stuff! The fundamental problem with our calendars is that we want a calendar year to be a particular number of days, but the actual time it takes the Earth to go around the sun (one year) isn’t an even number of days. It actually takes 365.2422 days, which makes every calendar-maker’s life very challenging.
To deal with that messiness, our current calendar, which is called the Gregorian calendar, uses leap years. You know about those; we just had leap day in February, after all. Once every four years, we add a day to the calendar, on February 29. But if you’ve done your math, you’ll know that adding one day every four years would take the average calendar year length to 365.25 days, which is a lot closer to the real length of a year, but not quite there.
In order to make up the difference, or at least get a little bit closer, there are a couple of other wrinkles to the Gregorian calendar. For the years ending in 00 that fall when we would normally add a leap day, we skip it instead. The only times when those years do have a leap day is if that year is divisible by 400; so, the year 2000, 1600, 1200, and so forth.
All these adjustments help our calendar to be more accurate over long periods of time. However, in any given year, our calendar doesn’t find the sun in exactly the same position it was in on that same date the year before. That’s true for the equinox as well, which is just one moment in time, pegged to the sun’s position rather than the position of a clock’s hands. In a normal year, the equinox inches forward compared to the year before. But on leap years, it gets bounced backward. That's why it stays roughly around the same date, on one of those three March days, but it can end up happening at any time on any of those days.
Because we recently hit one of the rarest Gregorian calendar quirks of all (the leap year in the year 2000), we are bounced back about as far as we can get. That's why the equinox will happen on March 19 in 2020, and will continue to land on this date or March 20 for decades. It won't fall on March 21 again until the year 2101!