Magic Morning is only available in
Disneyland and only the attractions in Fantasyland and Tomorrowland are open. It really depends on what attractions you want to do. Peter Pan and Hyperspace Mountain are likely to be the most popular attractions during MM. If you're at the front of the MM crowds, I would probably head straight to Peter Pan. If you're towards the middle or back and there is already a long line at Peter Pan when you get to Fantasyland, I would do the other attractions. If you're not into Peter Pan, I would probably opt for Hyperspace Mountain or some of the Tomorrowland attractions.
In terms of days, all days are going to be busy during the time when you're there. I would opt to do MM as early in the trip as possible because I usually find my family has the easier time getting up for it earlier in the trip.