A classic of the genre on far future planets, after the collapse of a star-spanning empire and the beginning of a new rise.
Tiamat, the main planet, has a culture of tech-loving Winters and living among nature Summers, and an interchange of power between them on the change of seasons -- every 150 years. The off-worlders have returned, but they keep many technologies secret -- in part because Tiamat has the secret of long life, found in the blood of the sea-dwelling mer.
The Winter Queen Arienhod want to change that. She makes plans for the Change by having clones of hers implanted in women who will raise them as Summers.
One turns out well: Moon. The book follows Moon through many events leading to the Changes. It includes her love for her cousin Sparks (actually conceived during the festivities, as her mother believes Moon was); the effects of the FTL drive on time; why sibyls, deemed superstitious nonsense by the Winters, are found off Tiamat; Arienhod's hatred for a police woman and her plots against her; the secrets of the mer; captives of a robber band, and more.
On re-reading it, I found a few coincidences a bit much, but they kept the plot going. Also, you can indeed follow Hans Christian Andersen's -- if you watch carefully.