Let's see...so many to choose from.
The Feast Day of St. George, patron saint of England (who certainly did not exist)
Anniversary of Shakespeare's death (too late for "medieval")
Anniversary (supposedly) of Shakespeare's birth (too late, and really just wishful thinking)
Death of Ethelred the Unready
Founding of the Order of the Garter in 1348 by Edward III.
The story of the founding of the Order is well-known and unverified, told by Froissart, who loved court stories but wasn't present at the time (and was only a child). It tells us that the Countess of Salisbury lost her garter while dancing at a court ball, to the discourteous amusement of the guests. Edward III gallantly picked up the garter and handed it back to her, saying "Honi soi qui mal y pens."["Shame to him who thinks evil."] This, supposedly, inspired him to create a chivalrous order named for the Garter. The Countess of Salisbury in question might have referred to Joan of Kent, who later became his daughter-in-law when she married the Prince of Wales (Edward, "the Black Prince"). Otherwise, it was Joan's mother-in-law, Catherine Montacute, whose husband was the 1st Earl of Salisbury. Later rumors were that she was the focus of inappropriate affection from Edward.
Other sources claim that Richard Lionheart had his soldiers tie garters around their legs in some acknowledgement of St. George, and that Edward wished to evoke both Richard and George when founding the Order.
Whatever the case of its founding, it is the most exclusive and prestigious Order in England and Wales, limited to the king, the Prince of Wales, and 24 additional knights. The original list is a Who's Who of political power in mid-14th century England—although some of them had to be knighted in 1348 to be able to receive the honor of the Order. "Ladies of the Garter" were also appointed, though without the prestige accorded the Knights of the Garter. King Henry VII ended this practice, but King Edward VII named his wife a Lady of the Garter, as did King George V and King George VI.
Queen Elizabeth II is an ex-officio member of the Order.