When is Christmas celebrated?

You must have noticed that not all Christian e-twinning partners Orthodox or Catholic, celebrate Christmas the same dates. You must have wondered why does that happen. In Greece, Italy, Spain and Poland, we celebrated Christmas on the 25th December while Serbians, Ukrainians and many others, celebrate Christmas on the 7th January which is 13 days later. By now you might have an idea why. If not, let's find out. 

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In 1582 the Gregorian calender was introduced and adopted by Spain, Italy, Poland and many other countries. In Greece it was adopted in 1923, in Turkey in 1926, Ukraine in 1918 and Serbia 1919. Until then they used the Julian calender. The reason why it changed is beause it didn't reflect properly the actual time its takes Earth to circle once around the Sun (tropical year). The Gregorian calender uses a more accurate rule to calculate the leap years. So they had to remove a number of days from the old calender. The main difference lies in the way they calculate the leap years. The Eastern Orthodox Church is still using the Julian calender while the rest use the Gregorian. So this is why although they are all Christians the dates they celebrate Christmas and Epiphany diverges.