In 1662 William Schellinks, a Dutchman visiting London, was surprised to see those born in Wales put a leeks in the bands of their hats. He learned that this was in memory of a battle won by the Welsh on Saint David's Day in which leeks had been worn to distinguish them from the enemy. 'So His Majesty and many more great Lords and gentlemen, common people, and even lackeys, coachmen, porters, and all kinds of riff-raff and layabouts wear one on their hats'.
'We saw some countryfolk carry such large leeks on their hats that their heads hung almost sideways because of them. And so on this day the Welshmen are greatly teased by the English, not only by calling after them Taffey, Taffey or David, David, but also by hanging out all kinds of dolls and scarecrows with leeks on their heads, and as they celebrate the day with heavy boozing, and both sides, from the ale, strong beer, sack and claret, become short-tempered, obstinate, and wild, so it is not often that this day goes by without mishaps, and without one or the other getting into an argument or a blood fight'.
(Taken from 'The Oxford Companion To The English Year', 1999.