Depending upon which definition you use, there are actually two different dates that the mark the first day of spring.
1 March 2018 is the first day of the meteorological spring
The astronomical spring begins on 20 March 2018 with the occurrence of the spring equinox as the days become longer than the nights
Spring in the meteorological calendar is the season beginning in March and ending in May. Astronomically, spring typically starts on the day of the vernal (or spring) equinox which falls around the 20 March in the Northern Hemisphere. The equinox refers to when the sun's path crosses the equator and the day and night are of equal lengths lasting around 12 hours each.
The beginning of spring can be one of the most recognisable transitions between the seasons as the days begin to lengthen and the temperatures warm. Sometimes without even knowing it we often define the seasons phenologically. This refers to biological indicators, most notably the blossoming or emergence of certain plants, or the activities of certain animals. The very origin of the phrase vernal equinox also alludes to this with 'vernal' coming from the Latin vernus meaning bloom.
The weather in the UK during spring is often calm and dry due to the Atlantic losing heat during autumn and winter, leading to less heat and moisture being transferred to the atmosphere. The sun is high in the sky during spring meaning temperatures can rise in the day but stay cool at night due to the moderating effect of the ocean temperature.
Temperatures seen in the UK during spring are largely influenced by latitude, with northern parts of the UK such as Scotland seeing cooler temperatures compared to the lower latitudes and the southern UK experiencing warmer temperatures.
all information from met office and goggle seaches