We’re not sure when Extra Magic Minutes will start officially.
Right now, you can re-optimize your plan as soon as the park opens, even if it’s before official opening. That’s the new feature that @PrincipalTinker mentioned.
Within a couple of weeks, you’ll be able to specify a start time up to 30 minutes before official opening on your plans, even for future plans. So you’ll be able to say “I know the park is opening 30 minutes early, so start my plan then.”