The CA website says you must be totally or partially unemployed when you file. Chefgrille has given you some incorrect information, in that you don t have to wait until the next quarter to apply.
I filed for CA unemployment once when I was laid off (yes, I knew the lay-offs were coming and I was already looking for another job). I filed the day after I got the notice and was denied. It goes by quarters. I missed that quarter and had to wait 3 more months to try to refile, then I got it.
Ideally if you knew that date on which you will be unemployed... you would be looking really hard for a new job to start then or soon after, even if it is just a low-wage unskilled job to get to buy till you find something better!
You cannot file ahead of time. That would be foolish if the state allowed that to happen.