First Day of Spring - Spring Equinox

There are two equinoxes every year – in March and September – when the sun shines directly on the equator and the length of day and night is nearly equal. Seasons are opposite on either side of the equator, so the equinox in March is also known as the "spring equinox" in the northern hemisphere. However, in the southern hemisphere, it's known as the "autumnal (fall) equinox".

On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an "equinox", derived from Latin, meaning "equal night".
However, even if this is widely accepted, it isn't entirely true. In reality equinoxes don't have exactly 12 hours of daylight. Firstly, because of the size of the sun, the top of the disk rises above the horizon (constituting 'sunrise' which is the start of 'daytime') when the center of the disk is still below the horizon. Secondly, Earth's atmosphere refracts sunlight which means that an observer can experience light (daytime) even before the first glimpse of the sun's disk has risen above the horizon. To avoid this ambiguity the term equilux is sometimes used in this sense.

The March equinox occurs the moment the sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north. This happens either on March 19, 20 or 21 every year. On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth's axis tilts neither away from nor towards the Sun.

Equinoxes – along with solstices – have been celebrated in cultures all over the world for as long as we have written history. One of the most famous ancient Spring equinox celebrations was the Mayan sacrificial ritual by the main pyramid in Chichen Itza, Mexico.

The main pyramid – also known as El Castillo – has four staircases running from the top to the bottom of the pyramid's faces, notorious for the bloody human sacrifices that used to take place here.
The staircases are built at a carefully calculated angle which makes it look like an enormous snake of sunlight slithers down the stairs at the precise moment of the equinox.