Yes, SOLSTICE is a Scrabble word!

Letter values in Scrabble US: S 1, O 1, L 1, S 1, T 1, I 1, C 3, E 1.

Is it a valid word?

SOLSTICE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

either of the two times of the year when the sun is at its greatest distance from the celestial equator