Today is the spring equinox in the northern hemisphere. It's also the equinox in the southern hemisphere, but it's the autumn one there. The equinoxes are the two points in the year that are halfway between midwinter and midsummer; halfway between the shortest day and the longest day. So today, day and night are exactly the same length, as long as you define "day" as "the time when the midpoint of the sun is above the horizon" and "night" as "the time when the midpoint of the sun is below the horizon". Most of us think of the day as beginning when any part of the sun is above the horizon and ending when every part of the sun dips below it, so in practice the day is slightly longer then the night on the equinox.
I imagine that ancient people, noticing that the midday sun was getting lower and lower in the sky as winter progressed, may have been afraid that this year it might carry on getting lower and eventually disappear altogether. Maybe they were very relieved after midwinter passed and they noticed the days lengthening again. Perhaps it seemed to them like a battle between light and darkness, between day and night, warmth and cold, life and death. At midwinter it must have seemed as if the darkness was winning the battle, but then the light began to fight back. By the equinox, the light was winning the battle - day is longer than night once again and all of nature is coming back to life. Plants are starting to grow again, animals are active and breeding. Hurrah for the force of light and life and everything that is good for we humans.
So it's no surprise that the date of midwinter is close to the feast of Christmas, when we celebrate the birth of the son (sun). He is small and weak at that point, but he is growing and he brings hope for the future. Other cultures past and present have important celebrations at that time of year which include ideas about light and life and warmth. Similarly, the date of Easter is defined in relation to the equinox. And other cultures have celebrations at this time which are all about rebirth, renewal, and the triumph of life over death. It's all the same celebration. It's all the same idea. It's built into us because we are alive and we live on a planet which has seasons. Maybe you're not religious yourself and don't celebrate Easter. Maybe you didn't know that today is the equinox before you read this post. But you knew the flowers are all coming out, didn't you? You noticed that the mornings aren't so dark, and the evenings are getting longer. It's good to be in tune with the changes of the year. Happy equinox.