A greeting which became a toast thought to be Viking in origin and incorporated into English.
The word came to be used for drink for the toast which came to be specialised spiced ale or wine and seasonalised around what is now Christmas time. waes hael is answered by drinc hael.
Eventually, the idea of Wassailing came to embody a celebration at a time of year involving drink.
In the West Country, wassailing is associated with apple trees, cider and firing guns into the apple trees. Apple trees would be sprinkled with wassail (the drink) and either guns fired or pots and pans clashed to make a noise supposedly to drive evil spirits away from the tree. The largest tree would have cider-soaked bread placed in the branches and a wassail song would be sung around it.
Elsewhere it involves either a common drinking vessel or visiting or both.
Carolling was a popular Wassail custom, wishing good cheer and being rewarded with money or food. This derives from the visiting custom although it does not now involve a communal drinking vessel.