Wearing the leek on St. David's day. Mr. Brady says St. David
caused the Britons under King Cadwallader to distinguish themselves by
a leek in their caps. They conquered the Saxons, and recall their
victory by adopting the leek on every anniversary (March 1st). (
Shakespeare makes out that the Welsh wore leeks at the battle of Poitiers, for Fluelleu says:
“If your majesties is remembered of it, the Welshmen did good service in a garden where leeks did grow, wearing leeks in their Monmouth caps, which, your majesty know, to this hour is an honourable badge of the service, and I do believe your majesty takes no scorn to wear the leek upon St. Tavy's Day” —Henry V. iv 7.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894