How did "September" shift from 7th month to 9th month of a year? (and same for October, November, December) [duplicate]

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?

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.