The Dark is Rising sequence by Susan Cooper

Ever since I first read these books as a child (or teenager? Don't remember!), these lines have been engrained in my memory:

"When the Dark comes rising, six shall turn it back;

Three from the Circle, three from the track;

Wood, bronze, iron; water, fire, stone;

Five shall return, and one go alone. 

Iron for the birthday, bronze carried long;

Wood from the burning, stone out of song;

Fire on the candle ring, water from the thaw;

Six Signs the circle and the grail gone before. 

Fire from the mountains shall find the harp of gold

Played to wake the Sleepers, oldest of the old;

Power from the green witch, lost beneath the sea;

All shall find the light at last, silver on the tree."