The vernal equinox
Asronomical Spring starts today in the northern hemisphere.
At 1028 UTC (~GMT) on 20 March 2017, the Sun crosses the celestial equator going northwards - the vernal equinox for the northern hemisphere and, in astronomical terms, the start of Spring for northerly latitudes. It will reach its highest northerly latitude at 0424 on 21 June, the summer solstice, and return to the southern hemisphere at the autumnal equinox, at 2002 on 22 September.
The times of such events move forward ~6 hours each year due to Earth taking ~365.25 days to circle the Sun - with a leap-year inserted every 4 years to maintain the link between dates and seasons.
Although this represents the start of astronomical Spring, in meteorological terms, Spring is the months of March, April and May.
. . and remember that, in the EU, civil time moves forward one hour at 0100 UTC on Sunday 26 March.
Further details and picture from the Royal Observatory link below . .