The first day of spring
If you've been watching where the sun sets since January, you will have noticed that the sun has slowly moved north along the western horizon. On Thursday at 12:57 p.m., the sun will climb back across the celestial equator into the northern sky, marking the vernal equinox and the first day of spring.
The tilt of our planet's axis causes the sun to appear to change its path across the sky as Earth orbits the sun. In the summer, the sun takes a long high path across the sky, and we get lots of sunlight. During winter, the sun is low in the sky, and its trek from sunrise to sunset is short. On the first day of spring and fall, the sun is in the middle of the sky and rises and sets due east and west. Because daytime is about equal to nighttime, we call these days' equinoxes.
The sun will continue its northbound journey until the summer solstice on June 21, then begin its trek south.