One month out: Start reaching out to your contacts. It's too early to start applying for assistant positions, because they tend to get filled within a week or two. With the exception of the bigger TV networks and studios that may have a longer hiring process, assistant candidates are usually expected to start as soon as possible. Instead of sending your resume out for jobs that will be long gone by the time you land at LAX, email your old internship supervisors and other contacts to let them know you'll be graduating soon and when you're planning on moving. You can also set coffees and drinks to catch up with them when you arrive. The most important thing at this early stage is to get back on people's radars.
Two weeks out: Try to make some new contacts. Ask for informational interviews with college alumni or other industry professionals who may be able to help you. If your contact is an executive, it can take some time to get a meeting on the books, so try to schedule one with the executive's assistant before you make your move. And make sure you network with assistants too, as well as anyone you used to intern with who may already be out in LA, or moving like you. You can also start looking at job postings to get a sense of the job market. Make sure your resume is in tip top shape, and use the postings to guide your bullet points.
One week out: If you're not too stressed out with packing and preparing for your move, you can go ahead and send your resume out up to a week before you arrive in LA. You may have to pass up an interview or two, but it's good to get the ball rolling. It's also okay to wait until your arrival to start the application process -- one week won't make much of a difference.
This timeline also applies to those who are moving to LA or NYC for the first time to start an entertainment career. You might not have previously established contacts, but during the month before your move, spend as much time as possible researching the industry and types of available jobs, and ask your friends and family if they know anyone who could meet you for coffee when you arrive.
If you want a job in the industry, you'll have to be very proactive, and although it won't be possible to lock down a position months in advance, reconnecting with old contacts and making new ones ahead of time will benefit you when you're finally ready to start getting those applications out.