There are six European countries where Christmas is celebrated 13 days later than Britain.

The reason why January 7 is the big day elsewhere all comes down to the historical use of calendars.

Different Christians went down separate paths when it came to their organising of dates – with Catholics and Protestants using the Gregorian Calendar, while Orthodox Christians still use the original Julian calendar.

Countries in the latter Orthodox group include Serbia, Montenegro, Ukraine, Russia, Belarus and parts of Bosnia & Herzegovina. They all stuck with the original solar calendar that was introduced by Julius Caesar in 46BC.

Sadly for Julius Caesar’s legacy, his calendar had a very slight miscalculation, meaning it has been growing slowly out of sync with the solar year as the centuries went by.

If Orthodox Christians continue using the Julian calendar, then they’ll be celebrating Christmas on January 8 in 2100.

Meanwhile, while the former group – which is the most common and includes nations like the UK, France, Spain and Germany – also originally used the Julian calendar, they changed to the Gregorian one in 1582, when Pope Gregory XIII introduced it.

The Gregorian calendar aimed to ensure that Easter – the Church’s most important festival – always fell around the spring equinox.

It was a direct response to the (albeit very minor) mathematical error that was made when the Julian calendar was introduced, meaning Easter would gradually fall later and later.

Even a small number of Orthodox Christians in East and South-East Europe have adopted the Gregorian calendar – including the Greek Orthodox Church and several Ukrainians too.

Nevertheless, the majority of Orthodox Christians have stuck with the Julian calendar. Orthodox Christians celebrate slightly differently too.

For a start, they don’t give gifts on their Christmas Day (January 9) but instead on 19 December, which is St Nicholas’s Day, or on New Year’s Eve on 31 December.

Indeed, they don’t tend to use the Gregorian calendar in their day-to-day lives, but rather just for holy festivals.