They are used to define the transitional periods between the seasons, so when winter changes to spring, spring to summer and so on.
They are key dates in the journey of the Earth around the Sun.
The position of the Sun in relation to the Earth affects how long days and nights are.
This year on 17 March the amount of day and night are completely equally split which means this day is called the equilux.
So what are equinoxes and solstices?
Equinox is basically the date twice a year when the amount of day-time we get is equal to the amount of night time we get.
This happens when the sun is positioned exactly above the equator.
Equinox occur twice a year, once around the 20th of March which is the spring equinox and again around September the 22nd, known as the autumn equinox.
The word equinox comes from two Latin words, equi which means equal and nox meaning night.
So, from the day of the spring equinox the day is longer than the night and from the day of the autumn equinox the night becomes longer than the day.
Simple right? Well there's a bit more to it...
So day and night are only NEARLY a totally equal length on the equinox.
The moment when day and night are truly of equal length occurs  a few days before the spring equinox, and a few days after the autumn equinox.
This is called the equilux... Still with us?
So the Solstice also occurs twice a year, we have a summer solstice around the 21 June in the Northern Hemisphere and a winter solstice around the 21 December.
The summer solstice is the day when the Northern hemisphere experiences its longest period of day light all year.
Winter solstice is the day when the Northern Hemisphere experiences its longest period of night time all year.
During the summer solstice, the sun in the middle of the day is at it's highest point in the sky of the year. During the Winter solstice this is reversed and the noon sun is at it's lowest of the year.
Some far northern countries such as Iceland and Norway experience continuous daylight for months. This is because the Northern Hemisphere is tilted towards the sun, this can also result in increased sunlight and warmer temperatures.
Hopefully that has shone some light (awful pun alert) on what equinox and solstice mean!
Equinoxes and solstices key dates in the calendar.