Wren Day, also known as “Wren's Day,” “Day of the Wren,” or “Hunt the Wren Day” (Irish: Lá an Dreoilín), is celebrated on 26 December, St. Stephen's Day in a number of countries across Europe. The tradition consists of "hunting" a fake wren and putting it on top of a decorated pole. Then the crowds of Mummers, or “Strawboys,” celebrate the wren by dressing up in masks, straw suits, and colorful motley clothing. They form music bands and parade through towns and villages. These crowds are sometimes called “Wrenboys.”

The wren celebration may have descended from Celtic mythology. Ultimately, the origin may be a Samhain or midwinter sacrifice or celebration, as Celtic mythology considered the wren a symbol of the past year (the European wren is known for its habit of singing even in midwinter, and its name in the Netherlands, "Winter King," reflects this); Celtic names of the wren (draouennig, drean, dreathan, dryw etc.) also suggest an association with Druidic rituals.

The Wren King is hunted at the Winter Solstice as a reenactment of the Celtic Myth that tells How Lleu Llaw Gyffes got his name, which is about the Goddess Arianrhod. The story is told in the Mabinogion, the collection of Welsh myth which was written down in the 12th century. This story is essential to an understanding of the custom of Hunting the Wren. Arianrhod is a major figure in this story and a Celtic form of one of the most important Proto-Indo-European Goddesses.

Originally, the Hunting of the Wren was celebrated on the Winter Solstice, specifically the day after “Long Night” (the longest night of the year) when people celebrated the birth of the new Sun on December 21st. This tradition incorporates Indo-European religion and custom in various ways, as the custom of “Burying the Wren” with a lament for the death of the Sun, and as the custom of Hunting the Wren which reenacts the myth of the celebration and naming of the new Sun, and finally as a performance in which the death and revival of the Sun is reenacted in the many Mummers’ plays. All of these customs and traditions had a social element as well as a religious one, so that they provide an opportunity for people to visit their neighbors, share food, tell stories and participate in singing and dancing.