If I understand correctly, the words September, October, November and December all come from French counterpart Septembre, Octobre, Novembre and Decembre, and ultimately from Latin septem, octo, novem and decem with a suffix -bre. Therefore I suppose their original meaning were from "7th month of a year" to "10th month of a year".

Apparently those words stand for "9th month" to "12th month" nowadays, so why and how did this happen?

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    Note that the meaning of the Latin roots is not changed regardless of where September, October, November, and December are in the calendar. While December might be the 12th month of the year, the name "December" does not mean "12th month of the year". – V2Blast Oct 25 '18 at 7:29
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    This is not a question about the English language per se. – Kris Oct 25 '18 at 9:45
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    @Kris it's about word etymology. – Ister Oct 25 '18 at 10:31
  • @Ister See answer by Laurel. – Kris Oct 25 '18 at 10:42
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    This is still a question about the origins of some English words. Just because the origins happen to predate English doesn't mean it's not a question about English. – BenM Oct 25 '18 at 11:03

According to the OED, the switch from month number 7 to 9 was made well before English was even a language:

The ancient Roman calendar (dating from around the mid 8th cent. b.c.) had ten months; c713 b.c. January and February were added to the end. In 153 b.c. the beginning of the year was moved to 1 January, when the Roman consuls were elected. This new ordering of the months remained when the Julian calendar was introduced in 45 b.c. and in the Gregorian calendar widely used today.

We only use the word "September" in English because it was brought in from Latin.

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  • So... does that mean "September" actually stands for the 9th month, despite the word "septem" means 7? – iBug Oct 25 '18 at 7:48
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    Possibly worth mentioning the commonly accepted - but incorrect - belief that the insertion of Julius and Augustus Caesar's months bumped the 7th-10th months up by two. Especially as the months that were replaced by July and August were 'Quintillis' and 'Sextillis' with obvious links to their 5th and 6th positions in the previous calendar. – mcalex Oct 25 '18 at 8:32
  • "... was made well before English was even a language" -- See my comment at OP. – Kris Oct 25 '18 at 10:41
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    Possibly worth explicitly adding that prior to "the beginning of the year was moved to 1 January" the beginning of the Roman year used to be March 1st, so September was the 7th month, October the 8th and so on. See also The Roman calendar, which also confirms that July/August were not inserted, just renamed from Quintilis and Sextilis. – TripeHound Oct 25 '18 at 11:08
  • @iBug No. September was the 7th month (when the year started in March). The name has been kept although it's "origin" is no longer correct. – TripeHound Oct 25 '18 at 11:11

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