The Twenty-ninth of May . . .
. . . which is Royal Oak Day, sometimes called Oak Apple Day.
The tune is an old English jig called "The 29th of May" and commemorates on this day the restoration of the Monarchy after the Cromwellian devastation. The 29th of May was chosen as it was the birthday of King Charles II.
A bit more here, together with some judicious links with further explication.
Labels: Regem honorificate.