Today is the spring or if you live “down under” autumn equinox which means that day and night have the same length.
It’s the beginning of spring or autumn depending on where you are and another part of the so-called Wheel of the Year starts.
It is a pagan idea that the year is organised in “nature” events in this case either the growing season or the harvest starts. When in Europe the high and mighty wanted to bring in Christianity they had quite some trouble to get rid of the old rituals and therefore many Christian holidays happen around the old pagan festivals but are just called differently.
Like Eostre (spring equinox, start of spring) is always just before Easter. Funny that the words sound quite similar as well.
This year it is even a little more important as there will be a so-called super moon (the moon is the closest to the earth) as well as an eclipse (the moon shifts itself between sun and earth and covers the sun).