This isn’t really the primary meaning of “wassail,” although it may have been a natural outcome of Christmas drinking ceremonies.
The primary meaning it to wish someone good health in a toast at a winter holiday drinking celebration.
http://www.dictionary.com/browse/wassail?s=t
I remember reading something about the etymology of the word that mentioned that the origin of the phrase “hale and hearty” is related.
We used to sing the Wassail song as children at Christmas. Never knew its meaning but I knew it had to do with drinking. Pretty cool...as compared to kwanzaa or other contrived BS!