I have wondered before why the months on our calendar were September, October, November, December, when “Oct” as a prefix usually means 8. “Sept” usually means 7, “Nov” serve like the prefix for nine, “Dec” – serves as the prefix for 10. However, we know our calendar says September is the ninth month instead of the 7th. October is the 10th month, instead of the 8 (think like OCTagon); November is the 11th month for us, and not the 9th. And December (think DECimeter) is the 12th for our modern accepted calendar, not the 10th month like “Dec” would suppose.

I found an article type response-webpage, that gives some insight as to why this is so. It really is like a forum of people responding, but I had trouble looking up this topic in the sense of what keywords to use in the search, so for now, this is the web info I have from the UK. Enjoy!
https://www.theguardian.com/notesandqueries/query/0,,-82486,00.html
Query is “Why is September the ninth month when it means 7, and likewise for October, November, December…?“