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At the start of January as you head back to school, the Christmas tree's packed away and mince pies are a distant memory.

But for millions of people around the world, 7 January is Christmas Day.

Orthodox communities in Europe, Africa and the Middle East - which make up more than 12% of all Christians - celebrate the big day weeks after most of the Western world.

It's because they use a different calendar, called the Julian calendar, to work out what day Christmas should be.

The origins of the difference over when to officially recognize the birth of Jesus Christ go all the way back to AD 325, when a group of Christian bishops met to agree a standard date for the church's most important holiday, Easter.

To do so, they decided to base it on the Julian calendar, a calendar which Roman ruler Julius Caesar had adopted in 46 BC - basing a year on the time it takes the Sun to go around the Earth.

However, the calculations overestimated the length of the solar year by about 11 minutes. As a result, the calendar and the solar year became increasingly out of sync as the centuries progressed.

The Gregorian Calendar was created by Pope Gregory in 1582 to fix some of the glitches in the Julian Calendar as astronomy became more accurate.

The majority of the Christian world adopted it and Great Britain changed to the Gregorian calendar in 1752.

However some believers - known as Orthodox Christians - felt this was wrong and stuck with the Julian Calendar.

By 1923, there was a 13-day difference between the two calendars, putting Orthodox Christmas 13 days after December 25, on 7 January.

All members of the Orthodox Church begin a period of fasting 40 days prior to Christmas. No meat is allowed during the entire fasting period.

After Christmas Eve mass on January 6th many people will gather with their family and friends to host a giant feast to celebrate the last day of their fast.

People in the Ukrainian and Russian Orthodox faiths will have 12 traditional dishes during Christmas Eve, representing Christ's apostles - they can include cabbage soup, baked apples, vegetable stew, and bread.

A well-known dish is Kutia, made of sheaf of wheat with honey and poppy seeds, which represents family and a good year of harvest.

Other Orthodox traditions include decorating the house and dinner table with a sheaf of wheat called a didukh to bring a good harvest and singing carols before the traditional 'holy supper'.