Oak Apple Day is an old English Folk Festival which commemorates the restoration of the English monarchy in May 1660. It is in fact Charles II's birthday and at that time was a public holiday.

It is known as Oak Apple Day in remembrance of the oak tree in which Charles II hid and slept. There are a number of such Oak Trees around where the Kind is said to have taken refuge at different times including one  just outside Bridport Dorset. There is still an Oak Tree on that spot today and I drive past it regularly. 

There are many stories of Charles II hiding in Oak Trees all over the country. The public holiday was formally abolished in 1859. It is still however kept as Founder’s Day at the Royal Hospital in Chelsea commemorating the founding of the hospital by Charles II in 1681.

Further Information

Traditionally oak leaves are worn until midday.  For more information see https://en.wikipedia.org/wiki/Oak_Apple_Day