Why September means 'seventh month' but is the ninth.
The early Roman calendar began in March and had only ten months; winter was left unnamed. September, October, November and December simply mean 'seventh', 'eighth', 'ninth' and 'tenth' month. When January and February were later added at the front, every name shifted by two places. July and August were only renamed later in honour of Julius Caesar and Augustus.
Useful: When a numbering doesn't fit, something was usually inserted in front of it later.
Source: en.wikipedia.org