The week between Christmas and New Years is the time when Mari Lwyd comes to visit, usually at night. Usually, six people act out the tradition. One member would hold a horse's skull, while some of the others carry a sheet over the group to give the appearence of the ghost horse. The remaining members of the group tie colorful ribbons around the skull and use them as reigns to lead the horse around.
While the towns people sing Christmas Carols and Welsh traditional songs, the horse walks through the town. When the horse entered a house or bar, the horse would try to attack the residents. The people leadeing the horse would restrain the horse from attacking children.
Most of the chaos, although portrayed as terrifying, was all in good fun. The guests were often rewarded with Christmas desserts to thank them for bringing "good luck" to the house.
Mari Lwyd in fiction
A children's book entitled Silver on the Tree has a part where the protagonists cross paths with the frightening Mari Lwyd.