I just had to look it up
Restoration Day, more commonly known as Oak Apple Day or Royal Oak Day, was an English public holiday, observed annually on 29 May, to commemorate the restoration of the English monarchy in May 1660. In some parts of the country the day is still celebrated. It has also been known as Shick Shack Day, or Oak and Nettle Day.
In 1660, Parliament passed into law "An Act for a Perpetual Anniversary Thanksgiving on the Nine and Twentieth Day of May", declaring 29 May a public holiday "for keeping of a perpetual Anniversary, for a Day of Thanksgiving to God, for the great Blessing and Mercy he hath been graciously pleased to vouchsafe to the People of these Kingdoms, after their manifold and grievous Sufferings, in the Restoration of his Majesty..."
The public holiday was formally abolished in the Anniversary Days Observance Act 1859, but the date retains some significance in local or institutional customs. It is, for example, still observed as Founder's Day by the Royal Hospital Chelsea, which was founded by Charles II in 1681.