A couple of weeks back (11th May), in answer to MariaAntonia’s #picoftheweek challenge, I featured an oak gall (The Very Gall). An oak gall is also known—incorrectly—as an oak apple. These are oak apples.

An oak apple: 3 May 2019

And another oak apple: 3 May 2019
It’s the gall’s immature form—the newly formed uterus, as it were—and it appears around this time of the year. Which explains why Restoration Day is more commonly known as Oak Apple Day.
However, for readers not steeped in British history, this might require some explanation.
Restoration Day, aka Oak Apple Day, dates back to May 1660 when Parliament passed into law—in its wonderfully convoluted language—that all British citizenry were henceforth to keep every 29 May as a public holiday.
Why that day? It was the day Charles II (son of the deposed and executed Charles I) rode triumphantly into London to restore the English monarchy and put an end to the Protectorate (think Roundheads, Cromwell, and the abolition of Christmas and all fun and games; the restoration of the fun and games was certainly worth celebrating). The oak was chosen because, it was said, after the Battle of Worcester (September 1651) Charles had hidden in an oak tree to effect his escape from the Roundhead army.
However, said public holiday was abolished in 1859. So, there you know. Is it worth reinstating? We could beat each other about the body with oak leaves!
Information gleaned from Wikipedia.