Why is Christmas celebrated on 7th January in Eastern Orthodox Churches instead of th

Why is Christmas celebrated on 7th January in Eastern Orthodox Churches instead of the 25th December?

Gregorian calendar - as opposed to Julian calendar. Eastern Orthodox are the first protestants against the Catholic church.

the catholic church has changed their calendar many times but not all other churches have wanted to go along with Romes changes

to take advantage of post-holiday sales

Pope Gregory instituted the Gregorian calendar some centuries after Christ. The Eastern Orthodox did not recognize the Gregorian calendar, they kept the older Julian calendar and still follow it. There is then a 13 day difference between the calendars and thus the different dates for the various festivals of the church.
Blessings in your journey!

Eastern Orthodox Christian Churches celebrate Christmass on 25Th December but some of the local Churches (like Russian or Serbian) follow the Julian calendar which is 13 days late.

The Gregorian calendar (from Pope Gregory) used in the West was a correction to the Julian calendar (from Julius Caesar). Over time, the incorrect calculation of leap years in the Julian calendar built up to the point where the calendar didn't match the seasons any longer. The Gregorian calendar corrected the calculation, and *added* some days (eleven, I think, at the time) to adjust the date to the position of the earth around the sun. The difference between the two calendars continues to build, and is at 13 days now.

Many Orthodox Churches have not changed to the Gregorian calendar for calculation of festivals in the church, so important dates occur 13 days later. That is, they have not yet reached December 25 on the Julian calendar, even though the civil calendar is (for me, at this moment) is Jan 3. They'll get to Julian-calendar December 25 on Jan 7 on the Gregorian/civil calendar.

This is the case for the Russians Serbians cop ts but not for most of us-
the Julian calender is 13 days slow but most of us use the Gregorian.