Xmas on 7th jan?

i was watching Buletin utama ( malaysia's 8pm news ) on tv3 while eating my mom's ayam masak oyster.

buletin utama is reporting the xmas celebration all around the world, and i wondering why is xmas celebration in russia is on 7th jan. not 25th disember?

the report says that orthodox christian celebrate it on 7th jan.
rc, sda, etc celebrate it on 25th dis

any explaination? i just want to know. gain more knowledge :)