December 26, 1925 – Turkey adopts the Gregorian calendar. Prior to 12/26/25, the Ottoman Empire used the Rumi calendar, a specific calendar based on the Julian calendar, but starting with the year of Muhammad’s emigration (Hijra) in 622 C.E. The Rumi calendar was officially used by the Ottoman Empire after Tanzimat beginning in 1839 – a period of reorganization and modernization in Turkey – and by its successor, the Republic of Turkey until 1926. It was adopted for civic matters and is a solar based calendar, assigning a date to each solar day.
Submitted here for your enjoyment today are stamps showing both calendars. The first, is a Turkish stamp overprinted with the Rumi Star and Crescent date of 1331 (Scott No. 302). The second is a 2007 stamp from Montenegro commemorating the 425th anniversary of the Gregorian calendar (Scott No. 155).