1 March represents the start of meteorological Spring.
In meteorological terms, Spring in the northern hemisphere covers March, April & May, with the other seasons following in 3-month periods.
But, astronomically, Spring starts at the Vernal Equinox, when the Sun crosses the celestial equator into the northern hemisphere - at 0349 UTC (~GMT) on 20 March. Astronomical Summer then starts at the Summer Solstice - 2243 UTC on 20 June, when the Sun is at its highest northerly latitude of 23º26’.
The times of these astronomical events are approximately 6 hours later each year so, to keep dates consistent year-on-year in the long-term, an additional day - 29 February - is added each 4th year (except at the turn of the centuary); this applies this ‘leap year’ of 2020.
In 2002, the EU 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, this year BST will apply from 29 March to 25 October. Whether UK continues to comply with this directive will need to be decided by Parliament.