To roam the neighborhood singing
Christmas carols, asking for a drink of
wassail in return. Wassailers bring their own
wassail bowls, which their
hosts fill with spiced
ale,
wine,
rum or
cider. In short, a
pub crawl around your own block.
Also, a kind of traditional song to be sung when wassailing, most of which are also about wassailing. A wassail typically praises the host in advance of the ale, blessing the lord and lady of the house for their imminent generosity, and praying God to reward them with a prosperous new year. You really lay it on thick in a good wassail -- ideally, your host should be so embarrassed by your profuse blessings that they get you good and drunk, then send you off to inflict your merry (i.e., sloshed) singing upon the folks next door.
Examples of traditional wassails include: