Wintersmith by Terry Pratchett
The older witches warn Tiffany Aching not to join in the dark Morris dance, but the soon-to-be-thirteen-year-old, who is usually so sensible, suddenly finds that she cannot resist her feet’s urging. Swept away in the heat of the moment, the young witch joins the magical dance before anyone can stop her.
She afterwards learns that she has danced with the Wintersmith. Winter himself becomes fascinated with Tiffany, whom he mistakes for the summer goddess. When the snow begins to fall, Tiffany discovers that every snowflake looks like her and that the Wintersmith is trying to become a man so that they can be together in a permanent winter.