The tradition of wassailing (also spelled wasselling) falls into two distinct categories: the house-visiting wassail and the orchard-visiting wassail. The house-visiting wassail, which traditionally occurs on the twelfth day of Christmastide known as Twelfth Night or Epiphany Eve (5 January), is the practice of people going door-to-door, singing and offering a drink from the wassail bowl in exchange for gifts; this practice still exists, but has largely been displaced by carol singing. The orchard-visiting wassail is the custom of visiting orchards in cider-producing regions of England and singing to the trees to promote a good harvest for the coming year. Notable traditional wassailing songs include "Here We Come a-Wassailing", "Gloucestershire Wassail", and "Gower Wassail".

Wassail (/ˈwɒsəl/, /-eɪl/ WOSS-əl, -⁠ayl) is also a beverage made from hot mulled cider, ale, or wine and spices, drunk traditionally as an integral part of wassailing.

According to the Oxford English Dictionary, the word "wassail" originated as a borrowing from the Old Norse salutation ves heill, corresponding to Old English hál wes þú or wes hál – literally meaning 'be in good health' or 'be fortunate'. It was initially used in the sense of 'hail' or 'farewell', without any drinking connotation. The English interjection "hail" is a cognate of the etymon of the second part of "wassail", and was probably influenced by the Old English phrase.

The expression later became part of the drinking formula "wassail ... drinkhail" which, the OED suggests, initially arose in England among the Anglo-Danes, and from there spread to the native population, being considered a specifically Anglo Saxon characteristic by the 12th century. The earliest record is of around 1140, in Geoffrey of Monmouth's telling of the Rowena story, which has wes heil ... drinc heil (or, in a variant reading, was heil). Later Middle English manuscripts have a variety of spellings, including wæs hæil, wæshail, wessail, washayl, washail, wesseyl, wassayl, wassaile, wassaylle, wessayle, whatsaile and whatsaill.

The second expression, "drinkhail", may derive either from Old Norse or Old English, again with a variety of spellings including drinkel, drincheheil, drechehel, drincheheil, drinceseil, drinqueheil, drinkeil and dringail.

The earliest example of the drinking phrases in a specifically English context comes from a manuscript of 1275, preserving a 12th-century text which has Old English: Þat freond sæiðe to freonde...Leofue freond wæs hail Þe oðer sæið Drinc hail. [That friend said to [the other] friend..., "beloved friend, wassail!"; the other said, "drinkhail!"]

By c. 1300, the sense had extended from a toast to the drink itself, especially to the spiced ale used in Twelfth-night and Christmas Eve celebrations. By 1598 it was being applied to the custom of drinking healths on those nights. Shakespeare's 1603 use of "Keep wassel" in Hamlet i. iv. 10 was the first record of the term's use in a more general sense of "carousal" or "revelling".

Here's to thee, old apple tree,
That blooms well, bears well.
Hats full, caps full,
Three bushel bags full,
An' all under one tree. Hurrah! Hurrah!