By what calendar do you live? Our current Western calendar is the end result of numerous changes in the past, but the whole idea that the Gregorian calendar came from an earlier 13 month calendar is somewhat of an untruth. Just ask Grok or any other AI to explain it to you.
While it is true that September was the seventh month at some time in the past, it became the ninth month because the months January and February were added at the start of the calendar that had only ten months before that change. And rather than renaming the months, the people that changed it decided to leave the names as is.
It is true that a thirteen month year has been proposed several times, and it may be better altogether, but all in all calendars is a big rabbithole if you really want to go down there.