When State Senator John Campbell resigned in order to be sworn in as a Member of Congress, he wrote and signed his letter on December 7, 2005.
The letter wasn’t printed in the Senate Journal until January 4, 2006. This has led some sources, including us until this evening, to record the date of his resignation as having occurred in January.
However, because Governor Schwarzenegger had called the Special Election back on December 15th, it’s clear that it was vacant prior to January.