Wikipedia "Mari Lwyd":
"The Mari Lwyd itself consists of a horse's skull that is decorated with ribbons and affixed to a pole; to the back of the skull is attached a white sheet, which drapes down to conceal both the pole and the individual carrying this device. On occasion, the horse's head was represented not by a skull but was instead made from wood or even paper. In some instances, the horse's jaw was able to open and close as a result of string or lever attached to it, and there are accounts of pieces of glass being affixed into the eye sockets of some examples, representing eyes. An observer of the tradition as it was performed at Llangynwyd during the nineteenth century noted that preparation for the activity was a communal event, with many locals involving themselves in the decorating of the Mari Lwyd.
The Mari Lwyd custom was performed during winter festivities, specifically around the dates of Christmas and New Year. However, the precise date on which the custom was performed varied between villages, and in a number of cases the custom was carried out for several consecutive nights. There is a unique example provided by an account from Gower in which the head was kept buried throughout the year, only being dug up for use during the Christmas season.

The custom used to begin at dusk and often lasted late into the night. The Mari Lwyd party consisted of four to seven men, who often had coloured ribbons and rosettes attached to their clothes and sometimes wore a broad sash around the waist. There was usually a smartly dressed "Leader" who carried a staff, stick, or whip, and sometimes other stock characters, such as the "Merryman" who played music, and Punch and Judy (both played by men) with blackened faces; often brightly dressed, Punch carried a long metal fire iron and Judy had a besom.

Wel dyma ni'n dwad (Well here we come)
Gy-feillion di-niwad (Innocent friends)
I ofyn am gennad (To ask leave)
I ofyn am gennad (To ask leave)
I ofyn am gennad i ganu (To ask leave to sing)

The Mari Lwyd party would approach a house and sing a song in which they requested admittance. The inhabitants of the house would then offer excuses for why the team could not enter. The party would sing a second verse, and the debate between the two sides – known as the pwnco (a form of musical battle similar to flyting) – would continue until the house's inhabitants ran out of ideas, at which time they were obliged to allow the party entry and to provide them with ale and food. An account from Nantgarw described such a performance, in which the Punch and Judy characters would cause a noise, with Punch tapping the ground to the rhythm of the music and rapping on the door with a poker, while Judy brushed the ground, house walls, and windows with a broom. The householders had to make Punch promise that he would not touch their fireplace before he entered the building, otherwise it was the local custom that before he left he would rake out the fire with his poker. In the case from Llangynwyd, however, there was no interplay between the householders and troupe, but rather the latter were typically granted entry automatically after singing the first verse of their song.

Once inside, the entertainment continued with the Mari Lwyd running around neighing and snapping its jaws, creating havoc, frightening children (and perhaps even adults) while the Leader pretended to try to restrain it. The Merryman played music and entertained the householders. The folklorist Iorwerth Peate believed that in recorded examples from Glamorgan it was apparent that the Mari Lwyd custom had become "indistinguishable" from the practice of wassailing, although added that there were still some examples of wassailing that did not involve the Mari Lwyd. He added that links between Mari Lwyd and wassailing were also apparent from recorded examples in other part of Wales, thus opining that Mari Lwyd represented a variant of the wider wassailing custom that was found throughout Britain."
  
The Mari Lwyd itself consists of a horse's skull that is decorated with ribbons and affixed to a pole; to the back of the skull is attached a white sheet, which drapes down to conceal both the pole and the individual carrying this device. On occasion, the horse's head was represented not by a skull but was instead made from wood or even paper. In some instances, the horse's jaw was able to open and close as a result of string or lever attached to it, and there are accounts of pieces of glass being affixed into the eye sockets of some examples, representing eyes. An observer of the tradition as it was performed at Llangynwyd during the nineteenth century noted that preparation for the activity was a communal event, with many locals involving themselves in the decorating of the Mari Lwyd.

The Mari Lwyd custom was performed during winter festivities, specifically around the dates of Christmas and New Year. However, the precise date on which the custom was performed varied between villages, and in a number of cases the custom was carried out for several consecutive nights. There is a unique example provided by an account from Gower in which the head was kept buried throughout the year, only being dug up for use during the Christmas season.

The custom used to begin at dusk and often lasted late into the night. The Mari Lwyd party consisted of four to seven men, who often had coloured ribbons and rosettes attached to their clothes and sometimes wore a broad sash around the waist. There was usually a smartly dressed "Leader" who carried a staff, stick, or whip, and sometimes other stock characters, such as the "Merryman" who played music, and Punch and Judy (both played by men) with blackened faces; often brightly dressed, Punch carried a long metal fire iron and Judy had a besom.

Wel dyma ni'n dwad (Well here we come)
Gy-feillion di-niwad (Innocent friends)
I ofyn am gennad (To ask leave)
I ofyn am gennad (To ask leave)
I ofyn am gennad i ganu (To ask leave to sing)

The Mari Lwyd party would approach a house and sing a song in which they requested admittance. The inhabitants of the house would then offer excuses for why the team could not enter. The party would sing a second verse, and the debate between the two sides – known as the pwnco (a form of musical battle similar to flyting) – would continue until the house's inhabitants ran out of ideas, at which time they were obliged to allow the party entry and to provide them with ale and food. An account from Nantgarw described such a performance, in which the Punch and Judy characters would cause a noise, with Punch tapping the ground to the rhythm of the music and rapping on the door with a poker, while Judy brushed the ground, house walls, and windows with a broom. The householders had to make Punch promise that he would not touch their fireplace before he entered the building, otherwise it was the local custom that before he left he would rake out the fire with his poker. In the case from Llangynwyd, however, there was no interplay between the householders and troupe, but rather the latter were typically granted entry automatically after singing the first verse of their song.

Once inside, the entertainment continued with the Mari Lwyd running around neighing and snapping its jaws, creating havoc, frightening children (and perhaps even adults) while the Leader pretended to try to restrain it. The Merryman played music and entertained the householders.^[9] The folklorist Iorwerth Peate believed that in recorded examples from Glamorgan it was apparent that the Mari Lwyd custom had become "indistinguishable" from the practice of wassailing, although added that there were still some examples of wassailing that did not involve the Mari Lwyd. He added that links between Mari Lwyd and wassailing were also apparent from recorded examples in other part of Wales, thus opining that Mari Lwyd represented a variant of the wider wassailing custom that was found throughout Britain."
  -Mari Lwyd Wikipedia arcticle
  Mari Lwyd - Wikipedia