when do master students start school in american universities, esp california un?

Answer 1

I'm not sure what the question is. Are you looking for the date on which classes start, or are you asking at what stage in one's career one starts a Master's degree. If the former, it is different for every university. Some have started already, while others don't start until late September. If the latter, the answer is that you need to finish a bachelor's degree first, but assuming that, you may start at any time.