This is the blossom of the hawthorn tree. It is also called may or mayflower, and the hawthorn tree is still sometimes called a may tree.
Mayflower was used in May Day celebrations before the eighteenth Century because, almost invariably, it was in flower on the first day of May, the traditional May Day. That was where it got its name. It was used to decorate the outside of houses during May Day celebrations, and arches of mayflower were paraded through villages to the maypole set up on the village green. The May Queen was crowned with a garland of mayflower.
But in 1752 England adopted the Gregorian calendar. Until then, we had been using the Julian calendar (established by Julius Caesar) and miscalculating by a day in every 128 years. By 1752 we were eleven days out. Parliament solved this by just skipping eleven days; that year September 2nd was followed by September 14th. This meant that the next May Day arrived before the mayflowers opened.
So, for a long time the may trees only occasionally bloomed in time for May Day celebrations but now climate change is slowly making up the lost time. As spring arrives earlier every year, the may blossom opens earlier as well. It is now, once again, very likely that the mayflowers are open on May Day.
More about hawthorn:
“Shed not a clout till May be out.”