1930 – Constantinople is officially renamed Istanbul

Although the name had been in use informally since 1453, in most contexts Istanbul was still Constantinople to non-Turks, and Kostantiniyye in most government contexts. After the fall of the Ottoman Empire and the formation of the modern Turkish Republic in 1923, the old name was gradually phased out.

The changeover was formalised on March 28, 1930, when the Turkish Postal Service Law came into force. All foreigners were requested to stop using the old names of Istanbul and various other Turkish locations. This was enforced by the post office’s refusal to deliver mail addressed to Constantinople, which drove acceptance of the new usage on pragmatic grounds.

Referenced in:

Istanbul Not Constantinople – The Four Lads
Istanbul Not Constantinople – They Might Be Giants