The Romans had twelve months and they even named January and February, it’s usually attributed to Numa Pompilius, second king of Rome sometime during his reign (715–672 BC) of the Roman Kingdom.
All covered in the link. The addition of January and February and later moving the new year from March to January is the reason Sept-Dec are no longer the seventh-tenth months. Not July and August, which were renamings, not additions.
Edit: I suppose my first comment should have specified early Romans. The way I wrote it could be read as all those changes happening after the Romans.
The Romans had twelve months and they even named January and February, it’s usually attributed to Numa Pompilius, second king of Rome sometime during his reign (715–672 BC) of the Roman Kingdom.
All covered in the link. The addition of January and February and later moving the new year from March to January is the reason Sept-Dec are no longer the seventh-tenth months. Not July and August, which were renamings, not additions.
Edit: I suppose my first comment should have specified early Romans. The way I wrote it could be read as all those changes happening after the Romans.