The Sovnarkom decree announcing the change of calendars.
February 13 1918, Petrograd–In keeping with the Russian Orthodox Church, Russia had maintained its use of the Julian Calendar, viewing the Gregorian calendar suspiciously as a Catholic invention. As a result of the Gregorian calendar’s occasionally-missing leap days, the dates in Russia had slipped thirteen days behind the Gregorian over the centuries. The February and October Revolutions in 1917 took place in March and November by the Gregorian calendar.
The Bolsheviks, who had already been anathematized by the Russian Orthodox Church for their explicitly anti-religious views, held no special affection for the Julian Calendar. Modifications to the calendar were also not uncommon for European revolutions; the French had tried a far more radical change in the 1790′s. On January 24 in the Julian Calendar, Sovnarkom decreed an upcoming switch to the Gregorian Calendar, to bring them in line with the rest of Europe’s workers. The final day of the Julian Calendar in Russia was January 31 (February 13 in most of the rest of Europe), which was followed by February 14. To ease the transition, the Julian date would be written in parentheses after all dates until July 1.
Several other belligerents had switched their calendars during the war, even without a revolution–presumably to coordinate with their allies or as part of a break with Russia. Bulgaria had switched in November 1915, shortly after entering the war; Turkey had aligned their days and months with the Gregorian calendar in March 1917 (though they continued to mark their years in the Islamic fashion). Serbia and Romania would continue to use the Julian calendar until March 1919, and Greece (whose “November Events,” the Noemvriana, took place in December by the Gregorian reckoning) until February 1923.
On a more personal note, this author is especially glad that the Bolsheviks switched the calendar, as the difference has caused no shortage of headaches; there has definitely been a few last-minute scrambles when the author realized an about-to-be posted event actually occurred thirteen days later.
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