The badnjak (Cyrillic: бадњак, ), also called veseljak (весељак, literally "jovial one" in Serbian), is represented by three types of objects in Serbian Christmas celebrations. The oldest type is a log brought into the house and placed on the fire on the evening of Christmas Eve, much like a yule log in other European traditions. The tree from which the log is cut, preferably a young and straight oak, is ceremonially felled early on the morning of Christmas Eve. The felling, preparation, bringing in, and laying on the fire, are surrounded by elaborate rituals, with many regional variations. The burning of the log is accompanied by prayers that the coming year brings food, happiness, love, luck, and riches. The log burns on throughout Christmas Day, when the first visitor strikes it with a poker or a branch to make sparks fly, requesting that the family's happiness and prosperity be as abundant as the sparks. Another type of the badnjak that has developed among the Serbs has mostly replaced the traditional log, whose burning is usually unfeasible in modern homes. It is a cluster of oak twigs with brown leaves attached, with which the home is decorated on the Eve.
The third type of the badnjak is represented by felled oak saplings which the Serbs use in their public celebrations on Christmas Eve. The Serbian badnjak tradition was originally a private affair conducted within the family, but since the early 20th century it has also been celebrated more publicly. Before World War I, soldiers of the Kingdom of Serbia developed the custom of laying a badnjak on a fire in their barracks. In the succeeding Kingdom of Yugoslavia, the military badnjak ceremony was further elaborated and standardized in army service regulations, but the tradition ended at the outbreak of World War II. Since the early 1990s, the Serbian Orthodox Church has, together with local communities, organized public celebrations on Christmas Eve in which the badnjak plays a central role. Parishioners festively cut the sapling to be used as the badnjak and take it to their church, where it is consecrated by a priest before it is ceremonially placed on a fire built in the churchyard.
The festive kindling of the badnjak commemorates the fire that—according to Serbian folk tradition—the shepherds of Bethlehem built in the cave where Jesus Christ was born, to warm the Baby Jesus and his mother throughout the night. The badnjak may also be seen as a symbol of the cross upon which Christ was crucified, the warmth of its fire symbolizing the salvation which, in the Christian belief, the crucifixion made possible for mankind. Scholars regard the tradition as inherited from the old Slavic religion. They interpret the badnjak as an incarnation of the spirit of vegetation, and as a divinity who dies by burning to be reborn, to whom sacrifices and prayers were offered for the fertility of fields, the health and happiness of the family. The burning symbolized sunshine, securing the vitalizing power of the sun in the coming year. The custom that a family brings a log into the house and burns it on Christmas Eve is attested among other South Slavic peoples, with similar names for the log. This custom has also been recorded in other parts of Europe.