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 equatorial into the northern hemisphere - at 2158 UTC (~GMT) on 20 March. Astronomical Summer then starts at the Summer Solstice - 1554 UTC on 21 June, when the Sun is at its highest northerly latitude of 23º26’.
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, this year BST will apply from 31 March to 27 October. Whether UK complies may need to be decided by Parliament.