The #EasternOrthodox Churches never adopted the Gregorian calendar (except in #Finland). While Orthodox-majority countries use the Gregorian calendar for civil purposes, many still use the #JulianCalendar for religious purposes. You have probably heard about #UkrainianRefugees celebrating #Easter and #Christmas on different dates to those of #Catholics and #Protestants; this is because the Ukrainian Orthodox Church still uses the Julian Calendar (for now).