Same thing for September, October, November, and December (Decimal System -- Twelfth Month!!!). All the months which refer to numbers refer to the wrong numbers ... and it's been that way for two thousand years!
We're the premier species on this planet, right? Well, it's a very small planet!!!
P.S. 2006 would be a good year to institute calendar reform. It starts on a Sunday.
All right, so what were the two months that were added to push December from 10 to 12?
Not quite 2000 years. Part of the Gregorian reform of 1582 was to move the beginning of the new year from March 25 (the Feast of the Annunciation) to January 1. In the Julian calendar September was indeed the seventh month after the beginning of the New Year, and so forth. March was the first (and last) month; April the second, and so on.
The Julian system was premised on the belief that the world was created on the same day that Jesus was conceived and the same day on which He was later Crucified.
That's because Julius and Augustus insisted on having their own months, complete with 31 days. That's also why February is short.
Actually, until the seventeenth century or so, the new year started with March, not January, so the naming system made sense up until then.