On 27 July 2023 a Local Council of the Orthodox Church of Ukraine, the body Constantinople had granted its own tomos of autocephaly in January 2019, voted to adopt the Revised Julian calendar, effective from the start of the new church year on 1 September that same year. The change moved every fixed feast thirteen days earlier: Christmas, kept for a century on the unrevised Julian date of January 7, was observed on December 25 for the first time that December, alongside the rest of the calendar's fixed observances. Parishes and monasteries that preferred to keep the old dates were told they could continue to do so, so the reform reached the church gradually rather than by decree alone.
The timing was not separate from the war. The decision followed Russia's full scale invasion in February 2022 by a little over a year, and it widened a break with Moscow that the 2019 tomos had already opened. The Julian calendar itself had come to be read in Ukraine as a marker of Russian church culture specifically, not simply of Orthodox tradition in general, and moving off it read as a further, deliberate step away from Moscow rather than as a change made for its own sake.
Where the six other Orthodox churches this atlas covers arrived at their calendar decisions across the 1920s, the Ukrainian church's decision is the newest, and its reasoning is explicitly the war.