This is the day that traditionally people wear oak apples or oak leaves pinned to them to remember that on May 29th King Charles ll returned triumphantly to London after the restoration of the monarchy in 1660.
The reason for the wearing of oak apples or oak leaves was to celebrate the King's narrow escape from capture by Cromwell's soldiers by hiding in an oak tree.
However, as the Wikipedia entry for Oak Apple Day notes:
These ceremonies, which have now largely died out, are perhaps continuations of pre-Christian nature worship. The Garland King who rides through the streets of Castleton,Derbyshire, at the head of a procession, completely disguised in greenery, which is affixed to a pinnacle on the parish church tower, can have little connection with the Restoration.
So, the Greenman, the Oak Apple Tree and return of the King (echoing the myths of Arthur's return to save the land), might, on this day all join in honoring Spring and the Merry Month of May.
The day shines, bright, go and enjoy!