How you define the first day of winter depends on whether you are referring to the astronomical or meteorological winter.
The day in our calendar that marks the first day of winter usually refers to the astronomical seasons which are a result of the Earth's axis and orbit around the sun. However, at the Met Office we often use a meteorological definition of the seasons. Let's take a brief overview of the difference.
Meteorological and astronomical seasons
Astronomical seasons are relative to the position of the Earth's orbit around the sun taking into account equinoxes and solstices. Meteorological seasons are instead based on annual temperature cycles measuring the meteorological state and coinciding with the Gregorian calendar to determine a clear transition and equal length seasons.
Meteorological winter season
The meteorological winter begins on 1 December 2018 and will end on 28 February 2019.
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, winter always starts on 1 December.
Astronomical winter season
The astronomical winter begins on 21 December 2018 and ends on 20 March 2019.
The astronomical calendar determines the seasons due to the 23.5 degree 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).
What is the weather usually like in winter?
The graphic below shows what the weather can be like during wintertime showing the wettest, warmest, sunniest, driest and coldest winters on record.