A white Christmas refers to the presence of snow at Christmas; either Christmas Eve or Christmas Day depending on local tradition. This phenomenon is most common in the northern countries of the Northern Hemisphere. Because December is in the middle of the Southern Hemisphere summer, white Christmases there are extremely rare, except in Antarctica, in the Southern Alps of New Zealand's South Island, and in parts of the Andes in South America. The Irving Berlin song, "White Christmas", sung by Bing Crosby from the film Holiday Inn, is the highest-selling Christmas single of all time and speaks nostalgically of a traditional white Christmas.