The final book in Susan Cooper's "Dark is Rising" sequence, Silver on the Tree. It's hard for me to give a good plot summary since I haven't read this, but the series got great acclaim, with (#2) The Dark is Rising a Newbery Honor book for 1974, and (#4) The Grey King winning the Newbery award in 1976, along with the Tir na n-Og Prize for the same year, and this book winning the Tir na n-Og Prize for 1977. The whole series is Arthurian-related, as some of the Old Ones are revealed in the great conflict between Light and Dark.