Oak Apple Day
Oak Apple Day was a public holiday in England on 29 May to celebrate the restoration of the English monarchy; it was formally abolished in 1859. An oak apple is caused by the larvae of a gall wasp, they are so called because the gall slightly resembles an apple.
Origin
Followers of the monarchy celebrated Charles II's birthday and his escape from the “Roundheads” on 29th May. It was called 'Oak Apple Day' in remembrance of the time when Charles hid in an oak tree following the Battle of Worcester on September 3rd, 1651 and evaded being captured by marauding soldiers. Due to the protection of this tree, Charles was eventually crowned king of Great Britain and Ireland on 29th May 1660.
Charles remembered the tree and declared the holiday in its honor, the parliament stating:
“Parliament had ordered the 29th of May, the King's birthday, to be for ever kept as a day of thanksgiving for our redemption from tyranny and the King's return to his Government, he entering London that day.”
Samuel Pepys (1660)
Today
Today, it is practice for monarchists to decorate the house with oak branches or wear an oak branchlet on May 29th. In All Saints Church in Northampton, for example, a wreath of oak-apples is laid at Charles II's statue. Its true origin is largely forgotten.
Sources
Rose, Jacqueline: Godly kingship in Restoration England. The politics of the royal supremacy, 1660 - 1688. Cambridge: University Press, 2011.