When does summer start?
Usually when we talk about the first day of summer we are referring to the astronomical summer which is defined by the Earth's axis and orbit around the sun.
This year the astronomical summer began on 21 June 2018 and ended on 23 September 2018.
The astronomical calendar determines the seasons due to the 23.5° tilt of the Earth's rotational axis in relation to its orbit around the sun. Both equinoxes and solstices are related to the Earth's orbit around the sun.
Solstices and equinoxes are considered to be the astronomical transition points between the seasons and mark key stages in the astronomical cycle of the earth. In a year there are two equinoxes (spring and autumn) and two solstices (summer and winter). The dates of the equinoxes and solstices aren't fixed due to the Earth's elliptical orbit of the sun. The Earth's orbit around the sun means that in early January, the sun is closest (known as perihelion) and in early July it is most distant (aphelion).
However, meteorologists are also interested in the beginning of the 'meteorological summer'.
The meteorological summer began on 01 June 2018 and ended on 31 August 2018.
The meteorological seasons consists of splitting the seasons into four periods made up of three months each. These seasons are split to coincide with our Gregorian calendar making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics. By the meteorological calendar, spring starts on 1 March.
The seasons are defined as spring (March, April, May), summer (June, July, August), autumn (September, October, November) and winter (December, January, February).