Either way works.......I have done several of these cruises and done it both ways. I like doing it on a free day, you get to experience a bit of the park without actually being in the park. Now for reservations here is what I recommend.
Use the Priority Seating calculator on this site to correctly determine the 90 day mark for the date you would like for your cruise. Then a few days before that time make a practice call. When you call ask them what time they have on their clock and then set your clock to it. On the day you need to call to make your reservation....and please remember that you need to do this precisely 90 days out at 7 a.m. est......begin dialing at 6:59 and 40 seconds. When you get the recording don't wait for it to play through offering you your options...immediately hit "4". That is the one you need. Be firm and polite when asking for an Illuminations Cruise. There are only 5 boats and they do book up fast, there is time for small talk after the boat is reserved.
You might even want to call when the office is closed so you can tell the difference between the 2 recordings. ONe says the office is closed and is different from the one they use when the office is open.
Good luck and enjoy your cruise!
Linda