Today the word wassail is rare and survives only in Christmassy traditions; it can mean mulled wine, or a booze up, or singing carols house to house. But originally it was not specially related to Christmas but just a drinking formula, e.g.
Me: Ves Heill
You: Drink Heill
Which means something like
Me: To Your Health
You: Drink To Health
Drinkhail has now sadly faded from the language, but wassail survives, a sort of verbal fly trapped in Christmas amber. Interestingly the drinking formula aspect is quite late. The expression Ves Heill existed before the Norman Conquest but there are no sources connecting it to the drinking formula I have described; it is just used in the sense of ‘be well’ or ‘farewell’. And the ‘hail’ form indicates Danish influence (in Old English it would have hal rather than heill). An interesting theory is that the drinking motif evolved among the Danish speakers in England e.g. in the reign of Cnut and then memed more widely. It’s lost its potency now though.