When is the first day of spring 2019 and the date of the Spring Equinox?

When is the first day of Spring in the UK? What is the Vernal Equinox?  Evening StandardSpring Equinox 2019 – what is the Vernal Equinox and when was the first day of spring?  The SunSpring Equinox 2019: 7 facts about the first day o