You should be able to book it right at 6am, but it didn't work out that way for me.
My 180 day was May 7th for an arrival Nov 3. For some reason the Fantasmic package was only available on the calendar until Oct 31. Not sure why, but it just recently opened up. We are only doing 1 evening in HS (plan to hop there either from our hotel or AK in the am), so for us it makes more sense to try and book a dinner for the package. MM was not available at all that day (I even tried earlier times just to see), and H&V had one opening at 6:15 for my party size. Right now the package is booking until Nov 19th.
If you don't get what you want on your 180 day keep checking the website. I also called on the 180 day morning and was able to get better times for a few things - maybe just luck but worth a shot.