Today, 21 December, is the astronomical start of Winter.
The Sun reaches its furthest southerly latitude of 23º26.1’ at 2223 UTC on 21 December - the Winter Solstice for the northern hemisphere and the start of astronomical Winter.
The Sun will re-cross the equatorial into the northern hemisphere at 2158 UTC on 20 March 2019 - the Vernal Equinox and the start of astronomical Spring.
But, in meteorological terms, northern Winter is defined as the months of December, January and February, with Spring following for the 3 months from 1 March.
From 2002, the EU has stipulated that all member states should adopt ’Summer Time’ between the last Sundays in March and October - the clocks to change at 0100 UTC. So, in 2019, BST will apply from 31 March to 27 October. Whether UK complies is open to question.