When you say "feb. 12 to 20" I assume you mean April 12 to 20 which takes you to Easter Sunday. The Easter crowds begin to set in the weekend before Easter and hit full force by Monday to Tuesday before, and then last until the Friday after Easter with some relief coming the Saturday after and by the Sunday after you are back to normal "spring break" crowds. As to those two weeks surrounding Easter:
1. You mentioned going before in summer. If that was the July 4 week, it approaches the Easter crowd. If it was another week, then the crowd you saw then would feel like a very light off season crowd in comparison to Easter. Same comparison to Easter for spring break weeks other those two Easter weeks (for the question about March 19 to 23 above, unless you have been at Easter and know how bad it can actually get, that time will feel quite crowded, which it is, but still quite manageable).
2. Those two Easter weeks are very comparable to Christmas week for the poster who asked above. Popular rides get 90 minute to 2 1/2 hour lines all day and evening with some relief only during parades. Test Track goes to 3 hours. Even Swiss Family Treehouse, the worst of the worst, gets an hour line much of the day. Walking at noon becomes difficult at MK as people are essentially elbow to elbow on the streets. You do get long hours like at Christmas. Easter has a slight advantage over Christmas because some of the park crowd is siphoned off by heavy attendance at the waterparks. Use fastpass to extent possible but even those run out for the day before noon for popular rides.
3. The waterparks tend to fill to capacity by 11:30 (and then open up again about 2:30). Those are also extremely crowded. Biggest problem is that the earliest arrivals seize all the chairs and chairs are near impossible to get by 11.
4. People actually start seizing front curb and other ideal viewing spots for the afternoon MK parade 2 hours before it begins. (Best is just to wait until close to start and find an elevated area off the front, like the stairs and porch at Town Hall).
5. Early entry can change during those weeks depending on expected crowds as they sometimes go to a system where they just open all the parks an hour early every day (whether they will do so is not determined until the time arrives and thus you should check for actual opening times when you get there).
6. PS's for dinner should be considered necessary particularly for park restaurants; many actually have no seating times available in the evening at all unless you have a PS and others will have 2 hour or longer wait without PS. Make PS's even for resort restaurants although you can usually still get most of those for something somewhat close to your desired time by calling the day before.
7. If you hit a park as early as allowed you can often get on a number of things without much trouble for about the first 45 minutes. Also, at Epcot and MGM, attractions other than Test Track, RnR, Tower of Terror, and Mermaid, can have fairly short lines after 3. Even at MK, rides like Pirates, Haunted Mansion, and some others can have short lines between 4 and 8 (people break for dinner and by 8 the full crowd returns to MK). Animal Kingdom will literally feel like a zoo all day but late afternoon you can get lucky and find some shorter lines. Most crowded times at any of the parks are about 10:30 to 3 (4 at MK). MK is the most crowded the entire time and it is even worse than that on Easter Sunday (and day before and day after Easter approach that Sunday crowd which often goes to capacity). The other parks tend to have their lighest (but still crowded) days of the week on Easter Sunday and day before.
6. Usually they have two Fantasmics at night (although budget cuts may affect that this year like last). For the first one, you need to get in line at least an hour early to assure getting a seat; for the second one you can walk in just before it starts as it never fills.
7. Universal and Sea World also get extremely crowded although they seem to have a somewhat longer (almost two hour) early morning safe period before the huge crowds set in.
