20 March is the spring equinox, marking the first day of spring in the astronomical calendar.
There are two methods to mark the start of a new season, meaning that the start of spring is recognised on two different dates.
In the meteorological world the year is broken up into groups of three months and the season is starts on the first. Meteorological spring is always 1 March.
Spring – (March, April, May)
Summer – (June, July, August)
Autumn – (September, October, November)
Winter – (December, January, February)
This is to make it easier to compare seasonal patterns between different years.
The second method is the Spring Equinox, which is part of the astronomical seasons:
Spring and autumn equinox – Day and night are equal.
Summer Solstice – The day is longer than the night.
Winter Solstice – The night is longer than the day.
On the equinox, the sun is shining directly over the equator and the day and night are equal lengths all over the globe. Following the equinox the days are longer than the nights in the northern hemisphere, and the nights are longer than the days in the southern hemisphere. For the autumn equinox it is reversed.
Whether you define spring meteorologically or astronomically, after 20 March 2017 it will be spring for everyone!