Deck the Halls

The melody to this popular carol is Welsh and dates to sixteenth century winter carol, "Nos Galan". The English lyrics, however, were composed by the Scottish musician Thomas Oliphant. The original lyrics clearly were intended to be a drinking song (see below). But the variant currently popular today first appeared in an 1877 issue of the Pennsylvania School Journal and eliminiates the references to drinking. 

Deck the hall with boughs of holly,
  Fa la la la la, la la la la
'Tis the season to be jolly,
  Fa la la la la, la la la la
Don we now our gay apparel,
  Fa la la la la, la la la la
Troll the ancient Yuletide carol,
  Fa la la la la, la la la la

See the blazing yule before us...
Strike the harp and join the chorus...
Follow me in merry measure...
While I tell of Yuletide treasure...

Fast away the old year passes...
Hail the new, ye lads and lasses!...
Sing we joyous all together...
Heedless of the wind and weather...

The original Oliphant lyrics are:

Deck the hall with boughs of holly,
'Tis the season to be jolly:
Fill the meadcup, drain the barrel,
Troll the ancient Christmas carol.

See the flowing bowl before us,
Strike the harp, and join in chorus:
Follow me in merry measure,
While I sing of beauty's treasure.

Fast away the old year passes,
Hail the new, ye lads and lasses:
Laughing quaffing all together,
Heedless of the wind and weather.

Later versions began to pluralize "halls" and change Christmas to "Yuletide". 

Here is a literal English translation of Talhairan's Welsh lyrics:

Goreu pleser ar nos galan,
Tŷ a thân a theulu diddan,
Calon lân a chwrw melyn,
Pennill mwyn a llais y delyn,

The best pleasure on new year's eve,
Is house and fire and a pleasant family,
A pure heart and blonde ale,
A gentle song and the voice of the harp