There are two things happening every year at this time - Easter Holiday and Spring Break. Many Spring Break periods are in the month of March no matter when Easter lands. Some Spring Breaks are scheduled around Easter every year (week before or week after). What does this mean? The earlier in the year that Easter lands, the more crowded the parks are going to be. The later, the lower the crowds.
We were in WDW the week after Easter in 2011 and the parks were busy, but not bad at all. Easter fell on 4/24 that year. The only Spring Breakers that we had to deal with were from schools like ours that always take off the week after Easter. Everyone else, colleges and schools, were back in school.
A few years before we went to WDW for a day before a cruise. It was late March, but Easter fell in March that year. The crowds were so bad that we left after only riding PoTC. That was the worst that I have ever seen WDW (never been at Christmas). We later found out that the park had closed due to the crowds, and stayed open 2 hours later than scheduled. Insane. Wouldn't go under those conditions again if someone else paid for the trip.