And it's not only that! Novem is Latin for nine, and Decem is ten, and yet the months are eleventh and twelfth respectively.
What's the origin of that? Why are the months called the way the are called? What caused this offset?
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The numerical names of months date back to the time when in the Ancient Rome the year began in March, so September was really the 7th month, October the 8th, and so on. January became the first month in 45 BC when Julius Cesar reformed the calendar introducing what is now known as Julian calendar. In those times July and August also had numeric names, Quintilis and Sextilis respectfully. See Roman calendar article on Wikipedia.