The anniversary of the Stuart restoration to the English throne (29 May) in 1660, when oak apples or oak leaves were traditionally worn in memory of Charles II hiding in an oak after the battle of Worcester.
Origin
Early 19th century; earliest use found in The Gentleman's Magazine.
Definition of Oak-apple Day in US English:
Oak-apple Day
noun
The anniversary of the Stuart restoration to the English throne (29 May) in 1660, when oak apples or oak leaves were traditionally worn in memory of Charles II hiding in an oak after the battle of Worcester.
Origin
Early 19th century; earliest use found in The Gentleman's Magazine.