What is the name of the Slavic winter solstice festival?

The name of the Slavic winter solstice festival is Koliada.

Koliada, also spelled Kolyada or Koleda, refers to a traditional Slavic winter festival and, more broadly, to Christmas-season rituals extending from Christmas toward Epiphany. Its older associations include the winter solstice, the return or honoring of the sun, and the symbolic rebirth of the year. Because customs varied across Slavic regions, Koliada is better understood as a festival period and ritual tradition than as one universally standardized holiday.

A major feature is kolyadovanye: groups of singers travel from house to house, perform seasonal songs, offer good wishes, and receive food, sweets, or money. Christian traditions later reshaped many Koliada customs, so modern meanings differ by language and country. In some places the word means Christmas itself; elsewhere it refers to caroling or related festivities. It should not be confused with Maslenitsa, a later pre-Lenten celebration, or Kupala, the better-known summer solstice festival.