Ukraine's parliament, the Verkhovna Rada, approved the measure — which also adjusts the dates of the Day of Ukrainian Statehood and the Day of Defenders of Ukraine — on July 14, with Zelenskyy formalising it on July 28.

Zelenskyy introduced the legislation on June 28 with an explanatory note that the change would "discard the Russian legacy of celebrating Christmas on Jan 7," the date for the holiday according to the Julian calendar, which the Russian Orthodox and other Eastern Orthodox churches use.

The Julian and Gregorian calendars — implemented in 1582 by Pope Gregory XIII and used by Western churches — differ by 13 days.

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In fact, writes Andrew McGowan of Trinity College at the University of Melbourne, the first mention of a date for Christmas Day (c. AD 200) and the earliest recorded celebrations (c. 250-300) were at a time when Christians were not borrowing heavily from pagan traditions.

Strangely, he says, the date of Jesus' birth may have been calculated from the date he died. He was believed to have been conceived and crucified on the same day, March 25, so the date of his birth was fixed at nine months later.

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