For the first time, Ukraine will celebrate Christmas on December 25th following the Western (Gregorian) calendar.
The country had so far followed the Julian calendar wherein Christmas fell on January 7, which Russia also follows.
However, Ukrainian President Volodymyr Zelensky changed the law in July, saying it allowed Ukrainians to "abandon the Russian heritage" of celebrating Christmas in January.
The move was aimed as a snub to Russia and the Russian Orthodox Church.
In a Christmas message issued on Sunday evening, Zelenskyy said all Ukrainians were now together.
Though Ukraine has been under the spiritual guidance of the Russian Orthodox Church since the 17th century, it cut ties in 2014 after Russia annexed Crimea.
The Orthodox Church of Ukraine (OCU) broke away from the Russian Orthodox Church and hence grew into the major denomination by taking several Russia-linked church buildings in moves supported by the government.
Ukraine also has a Ukrainian Orthodox Church (UOC), historically linked to Russia.
The church claims to have cut ties with Russia because of the war and keeps the January 7 Christmas date. Millions still follow the UOC and so will continue to celebrate Christmas on January 7.
The country’s third Orthodox denomination, the Ukrainian Greek Catholic Church, will also hold Christmas services on December 25.