I looked up the song Steve T wrote (see his message of 21 June 2013
I find it a bit strange that oak, which looses its leaves in winter (apart from the Holm Oak Quecus Ilex, which has some resemblance to holly), should be the winter king and holly the summer king. I would have thought it would be the other way about.

SOLSTICE SONG by Steve Thomason

For six long moons the Oak King’s ruled
His breath lay o’er the land
Now his day is done and his time is gone
And the Holly King rules the land
The Holly King rules the land
The Holly King rules the land
And the Holly King rules the land

At Yule so chill, the sun stood still
And the Oak King came to power
And he danced the way for the light of day
And the bud, the leaf, the flower
etc

In the ground the Holly King lay
Healed the wounds he’d won that day
As the days grew long, the Holly grew strong
‘Til the dawning of the May
etc

And the sun rose high in a brightening sky
Then in June the Holly King rose
At Litha he was the stronger tree
And the Oak King he laid low
etc

Now until the Yule the Holly King rules
And the days grow short again
But the Oak King’s strong and it won’t be long
‘Til the Yule brings back his reign

https://soundcloud.com/sthomason-1/a-solstice-song-oak-and-holly
"There is a cyclic battle between the Oak King and the Holly King. At Yule (winter solstice) the Oak King triumphs and presides over the waxing year. At Litha (summer solstice) the Holly King regains his crown and presides over the waning year. The tune is (more or less) the Candlelight Fisherman. This one is designed mainly for the summer solstice; I've got a different one for the winter solstice."