Christmas lights (also known as fairy lights, festive lights or string lights) are lights often used for decoration in celebration of Christmas, often on display throughout the Christmas season including Advent and Christmastide. The custom goes back to when Christmas trees were decorated with candles, which symbolized Christ being the light of the world. This custom was borrowed from pagan yule rituals that celebrate the return of the light of the sun as the days grow longer after solstice: the evergreen trees symbolising the renewal and continuance of life in dark times. The Christmas trees were brought by Christians into their homes in early modern Germany. Christmas trees displayed publicly and illuminated with electric lights became popular in the early 20th century. By the mid-20th century, it became customary to display strings of electric lights along streets and on buildings, Christmas decorations detached from the Christmas tree itself. In the United States, it became popular to outline private homes with such Christmas lights in tract housing beginning in the 1960s. By the late 20th century, the custom had also been adopted in other nations, including outside the Western world, notably in Japan and Hong Kong. It has since spread throughout Christendom. In many countries, Christmas lights, as well as other Christmas decorations, are traditionally erected on or around the first day of Advent. In the Western Christian world, the two traditional days when Christmas lights are removed are Twelfth Night and Candlemas, the latter of which ends the Christmas-Epiphany season in some denominations. Leaving the decorations up beyond Candlemas is historically considered to be inauspicious.