Oak Apple Day or, as it is also known, Royal Oak Day used to be officially celebrated as a holiday in England on 29 May to commemorate the restoration of the English monarchy, in May 1660. I believe that in some parts of the country, the day was also known as Shick-Shack Day or Arbour Day.
It was first declared as a public holiday by Parliament in 1660:
"Parliament had ordered the 29 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."
Sadly it was abolished (as a holiday, not as a celebration) in 1859 but the occurrences after the Battle of Worcester in September 1651, when the future Charles II of England escaped the Roundhead army by hiding in an oak tree near Boscobel House, are still widely celebrated in Shropshire and elsewhere to commemorate the return of our beloved Monarchy.
Have a Happy Oak Apple Day (and a restful Bank Holiday weekend).