The Date of Separation is the date that both parties understand the marriage is over. In other words, both husband and wife must understand that at least one of them considers the marriage over. This understanding is usually accompanied by a physical act such as moving out of the family residence, or moving into separate bedrooms. This is an important date because it marks the end of the community property accumulations and obligations. As of the Date of Separation, all earnings and accumulations are the separate property of that party. Likewise, debts incurred after the Date of Separation are separate property debts. If you believe you are separated but continue to live together as a married couple, or if you continue to conduct yourselves in a manner which may lead others to believe you are still married, your spouse may argue in court that you were not really separated. If their argument is successful, your earnings will be characterized as community property earnings to which they are entitled a 50% share, and debts they incurred while you thought you were separated will be characterized as your community property debts for which you are 50% obligated.
Make sure that you have a conversation with your spouse making it clear that the marriage is over, that you agree you are separated as of a specific date, and then be sure to back up your words with actions and documentation.
UPDATE!!!
As of July 2015, the California Court of Appeals determined that parties must physically separate, that is, live in separate residences, in order to establish a Date of Separation. If parties mediate or collaborate, they can agree to a Date of Separation other than the date one of them moved out, but lacking agreement and until the law is changed, someone must vacate the family residence in order to establish a Date of Separation. This new development in the law will undoubtedly create hardships for families that cannot afford the expense of moving out and for parties that want to stay in the family residence during the divorce process for the sake of children.