When the Gregorian calendar was adopted to replace the Julian calendar in 1752, eleven days had to be dropped from the calendar, thus drawing all the dates forward by eleven days, which is why the hawthorn does not usually blossom now on May 1. This makes 11 May Old May Eve, when the Manx fairies and witches are supposed to be particularly active In Ireland the Lunantishees fairies guard the blackthorn trees and will punish anyone who tries to cut its wood on this day.
Solitary hawthorns growing on hills or near wells were considered to be markers to the world of the fairies. Any human who slept beneath one, especially on May Eve, was in danger of being taken away to the land of the Sidhe. Hawthorn is so potently magical that it is forbidden to bring it indoors except at Beltane. The flowering of the hawthorn marked the opening of the summer season, the time when people could get out and about, and when young men and women could meet up. May was often considered the month of courtship and love. For this reason, and the fact that the scent hawthorn blossom is supposedly redolent of sex,  the hawthorn is associated with love-making. In ancient Greece the wood was used for the marriage torch, and girls wore hawthorn crowns at weddings.
However, while in some circumstances it was considered to be a tree of love, like other fairy trees, it was very unlucky to bring it indoors:
Hawthorn bloom and elder-flowers
Will fill a house with evil powers.
 Geoffrey Grigson: The Englishman’s Flora, Phoenix House, 1956