Extra Magic Hours are a perk which comes with staying on Walt Disney World property. Usually one park has early EMHs for the day and one park has late EMHs for the day. For example, Animal Kingdom might be open from 7am-8am for Extra Magic Hours and then open at 8am to all other guests and Magic Kingdom might stay open later for resort guests from 12am-2am. The hours and schedule depend on the season and the parks.
The best early hours are Animal Kingdom and the water parks, without a doubt. For Animal Kingdom, you can get all of the big rides (Dinosaur, Everest, Kali River Rapids) done in an hour usually and sometimes even more than once in a span of two hours if you use Fastpass + planning. The early morning is also a great time to go on the safari because it is not as hot (especially during the summer) and so the animals are more likely to be outside. The water parks are the most crowded mid day because everyone decides that they need a break from the theme parks and want to cool off. You can ride the Family Raft Ride in Blizzard Beach four to five times in a row with a short line in the early hour although there are a lot of steps to get to the ride! You can also get on most of the bigger rides with almost no lines so that when the park opens, you can float down the Lazy River and relax before grabbing lunch and heading back to the resort.
The best late hours on the other hand are definitely Magic Kingdom. Magic Kingdom is the park that is most crowded during the day because it is the most family friendly park. But during late hours it is usually at least a little easier to get onto rides. If the late hours go into the morning, such as until 2am, then this is even better because the crowds thin during the last hour. It is fun to walk around Magic Kingdom late at night and watch the shops and restaurants close up as the park closes. We took a picture of the clock the last time we left after late hours. We also got to be on the last ride of Big Thunder Mountain Railroad for the night which was an awesome experience. I can say that I have closed a ride for the night now. Don’t bother standing in line for hours during the day for a big ride like Space Mountain. If you are staying on property and can stay up late, then ride it later on. Sometimes it can get cold during the late hours though, especially in November so try to bring layers with you to the park or you might end up buying a Mickey sweatshirt.
The early and late hours for the other parks are great as well because they do give resort guests access into the parks when no one else can be there. Keep in mind that there are more resort guests than you think though! The above recommendations are based on experience and what my family tends to prioritize each trip but if you love Epcot, then you might want to make sure you make the Extra Magic Hours there. And yes, it is possible to stay out late in Magic Kingdom until 12:20 with Late Hours and be in Animal Kingdom by 7am for Early Hours the next morning!
Also, I have recently started participating in a Mickey Monday linkup run by run. geek. run(disney) and adventuresbykatie. The links to read other Mickey Monday blogs for this week are: http://www.rungeekrundisney.com/2014/09/coast-to-coast-challenge-mickey-monday.html and http://www.adventuresbykatie.com/lucky-fortune-cookery-disneys-california-adventure/
Please check out these two links to read other articles about Disney!