The March equinox occurs on March 20 at 10:29 Universal Time. People in the northern hemisphere usually call it the spring equinox. In the southern hemisphere, it’s the autumn equinox.
The June solstice is at June 21 at 04:25. This is the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere.
The September equinox is on September 22 at 20:02. In the northern hemisphere, this is usually called the autumn equinox. South of the equator, it is the spring equinox.
The December solstice is on December 21 at 16:29. For those north of the equator, this is the winter solstice. In the southern hemisphere, it’s the summer solstice.
Many people consider the beginning of the season the equinox or solstice is named for to begin on the day of the equinox or solstice. But really, spring weather has usually arrived at 30° north (the southern United States for example) a few weeks before the March equinox while latitudes farther north may be waiting several more weeks for warmer weather.
An equinox is when the day and night are almost at equal length almost everywhere in the world. The sun appears to be traveling over the equator. If you are far enough north or south–at one of the poles–you will actually see the sun travel around the entire horizon during the course of the day.
A solstice is when the sun appears as far north or south as it can be in the sky–23.44°. One pole or the other is tilted toward the sun and gets sunlight all day. The other is tipped away from the sun and is in darkness all day. After this moment, the sun’s apparent movement is in the other direction.
Earth’s seasons happen because of the axial tilt. As the earth goes around the sun, the earth’s rotational axis remains pointed almost exactly at the same distant points. However, we are given the illusion of our planet tipping one way and then the other relative to the sun.