How you define the first day of spring depends on whether you follow the astronomical or meteorological seasons.
This year, spring will take place between March 20 and June 21, if you use the astronomical method.
But if you follow the meteorological calendar it's already started – with the season spanning from March 1 to May 31.
There are two different methods used for determining the dates of the seasons, the astronomical or meteorological method.
The Met Office tend to use the meteorological seasons, these are based on the annual temperature cycle and the state of the atmosphere.
The meteorological seasons also co-ordinate with the calendar ad they consist of splitting the year into four three month seasons.
Using this system the seasons are defined as: Spring (March, April, May), Summer (June, July, August), Autumn (September, October, November) and Winter (December, January, February).
By this system, spring starts on March 1 and runs through until May 31.
The other system is to use the astronomical seasons.
These refer to the position of Earth's orbit in relation to the sun taking into account equinoxes (when the length of the day is exactly the same as the length of the night) and solstices (the longest and shortest days of the year).
If you want to get technical, the astronomical calendar determines the seasons by when the Earth's rotational axis is at a 23.5 degree in relation to its orbit around the sun.
By this system spring starts on March 20 and runs through until June 21.
The seasons are due to the Earth rotating around the Sun.
The way the Earth rotates means that certain areas of the globe are tilted towards the Sun while other parts are tilted away from it.
This means there are different levels of sunlight reaching each part of the globe, causing the seasons.
Source: The Sun