When holiday variation is present so that the dates of certain holidays change from year to year, the relatively automatic seasonal adjustment procedures may fail to extract the seasonal component from a series since the holiday effects are not confined to that component. Turkey, a predominantly Muslim country, constitutes a good example of moving holidays since the official calendar is Gregorian, based on the cycles of the Earth around the sun, while significant Islamic holidays are tied to the Hegirian calendar, based on the lunar cycles.
Moving Holidays and Seasonal Adjustment: The Case of TurkeyS. Boragan Aruoba and Emre Alper ,
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Review of Middle East Economics and Finance