Worn on May 29th. May 29th was the birthday of Charles II. It was in the month of September that he concealed himself in an oak at Boscobel. The battle of Worcester was fought on Wednesday, September 3rd, 1651, and Charles arrived at Whiteladies, about three-quarters of a mile from Boscobel House, early the next morning. He returned to England on his birthday, when the Royalists displayed a branch of oak in allusion to his hiding in an oak tree.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894