As
jina said, Magic Morning (MM) is only offered in
Disneyland on Tuesdays, Thursdays, and Saturday mornings. This also happens to be the same mornings that onsite guests get Extra Magic Hour (EMH) access to Disneyland. You can find more information here:
https://disneyland.disney.go.com/guest-services/magic-morning-early-admission/
Note that you have to pre-purchase your park tickets if you want to have the MM perk. If you wait to purchase the ticket at the Disneyland ticket booths, those tickets will not be Magic Morning eligible (my friend found out the hard way). Also, you only get access to one Magic Morning per ticket (so even with a three day ticket, you can only go in for one Magic Morning).
As mentioned, Magic Morning allows guests access to certain attractions (Fantasyland and Tomorrowland), shops (same, plus much of Main Street), and dining locations (mostly Main Street) one hour earlier than regular park guests. Unlike non-MM/EMH mornings, the ticket gates do not always open early, so they may not allow guests to begin to enter the park until MM/EMH starts. I was there last November and the ticket gates open right at 9:00 am (park opening was 10:00 am). If you are planning on going to MM, I would make an effort to get to the ticket gates early so you are near the front of the line to get in.
After having done MM back in November, I realized that unless your in the front of the line to get into the park, it's not necessarily worth it to try and ride Peter Pan. The line quickly grows to be 30 to 45 minutes, so if you get to Peter Pan and the line is already that long, I would strongly consider skipping Peter Pan and going for other FL (or TL) attractions. Peter Pan is going to have a long line all day long, so whether you do it first or later, doesn't necessarily matter. Other FL attractions will have lines that build-up during the day, but may be markedly shorter during MM. This didn't seem to be as big of an issue with the TL attractions (my friends rode Space and Star Tours in the time we spent waiting for Peter Pan).