Orthodox Christians in Russia, Greece, Ethiopia and other countries around the world are celebrating Christmas Day today.

The date is taken from the old Julian calendar, which was introduced by Julius Caesar in 46 BC as a reform of the Roman calendar. It was replaced by the Gregorian calendar in the majority of Western Europe around 500 years ago.

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Russian Cossacks, dressed in traditional uniform, guard the Kazan Cathedral in Stavropol, southern Russia, during an Orthodox Christmas serviceEduard Korniyenko/Reuters
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People march on the street during a religious procession to celebrate Orthodox Christmas in Tbilisi, GeorgiaDavid Mdzinarishvili/Reuters
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People walk in front of the Holy Trinity Cathedral during a midnight Christmas service in Tbilisi, GeorgiaDavid Mdzinarishvili/Reuters
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People attend a ceremonial burning of dried oak branches for Orthodox Christmas Eve in front of a church in Smederevo, SerbiaAndrej Isakovic/AFP
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Sviatoslav Mahulia, 12, sings carols while going door to door in celebration of Orthodox Christmas in Iltsi, UkraineBrendan Hoffman/Getty Images
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Nazar, Stas, and Vasyl Mahulia, aged 6, 5, and 7 respectively, are greeted while carolling door to door in celebration of Orthodox Christmas in Iltsi, UkraineBrendan Hoffman/Getty Images
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People dressed in traditional costumes sing Christmas carols in KievValentyn Ogirenko/Reuters
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Russian Prime Minister Dmitry Medvedev and his wife Svetlana attend a Christmas service in Christ the Saviour cathedral in MoscowKirill Kudryavtsev/AFP
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Russian President Vladimir Putin attends an Orthodox Christmas service at a local cathedral in the village of Otradnoye in the Voronezh regionAlexei Druzhinin/RIA Novosti/Kremlin/Reuters
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An actor wearing a mask depicting Russian president Vladimir Putin plays the role of King Herod in a living nativity scene in the Ukrainian city of LvivYuriy Dyachysyn/AFP
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Ukrainian President Petro Poroshenko and his wife Maryna meet traditional Christmas carol singers at a state residence in the settlement of Huta in Ivano-Frankivsk region, western UkraineUkrainian Presidential Press Service/Mikhail Palinchak/Reuters
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Believers attend an Orthodox Christmas service inside the Church of the Nativity in Krasnoyarsk, SiberiaIlya Naymushin/Reuters
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Orthodox priests attend the Christmas service in Christ the Saviour cathedral in MoscowKirill Kudryavtsev/AFP
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A member of the congregation lights a candle during a midnight mass vigil at a Russian Orthodox church in LondonDan Kitwood/Getty Images
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A priest prays during an Orthodox Christmas service in a church in the village of Sosnovka in KyrgyzstanVyacheslav Oseledko/AFP
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Palestinian Orthodox Christians attend a Christmas mass at the Saint Porphyrius church in Gaza CityMahmud Hams/AFP
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Egyptian President Abdel Fattah al-Sisi and Coptic Pope Tawadros II attend a Christmas Eve Mass at St Mark's Cathedral in CairoAl Youm Al Saabi Newspaper/Reuters
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Burning Yule logs and fireworks are seen in front of Sokolica church in Ravna Romanija, BosniaDado Ruvic/Reuters
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A man stands in St Sava Church in the village of Velika Hoca, Kosovo, following the Orthodox Christmas liturgyHazir Reka/Reuters
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Greek Orthodox Patriarch of Jerusalem Theophilos III blesses the congregation at the Church of the Nativity in the biblical West Bank town of BethlehemMusa al-Shaer/AFP
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Belarussians celebrate a Christmas rite in the village of Ysiag, east of MinskMaxim Malinovsky/AFP
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Servicemen of the Belarussian Interior Ministry's special force take part in a service at a military base in MinskVasily Fedosenko/Reuters