Yes, WASSAIL is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, S 1, S 1, A 1, I 1, L 1.

Is it a valid word?

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

a punch made of sweetened ale or wine heated with spices and roasted apples; especially at Christmas

celebrate noisily, often indulging in drinking; engage in uproarious festivities
propose a toast to

Also: make whoopie, make merry, make happy, whoop it up.