Is there a law about buying alcohol on your 21st birthday in California?

The Right Answer is:

You are allowed to buy alcoholic beverages the day before your birthday! Most stores, on the other hand, are completely unaware of this and will not sell to you until your birthday. For fear of being discovered selling to minors, some liquor outlets will not sell to you until the day after your birthday. Simply go to a pub and your friendly bartender will purchase the first one for you!