My Aunt and Uncle just went for spring break, and it was MOBBED! Crowds are always crazy during that time.
We went Easter week this year and it was not MOBBED in our opinion. We rode Pirates 3 times in a row with 10 minutes lines each at 5pm on Easter Sunday. It is all how you plan everything. We always make Dinner ADR's, go at opening, leave about noon for a 3- 4 hour break and go back and stay until 11pm'ish. We had the best trip this year. Weather is GREAT.
I am going the last week of August this year, and it is, of course, FREE DINING.
We have also gone the last week in Aug 2 times and the free dining is not worht the extreme heat and rain. At Easter the weather was perfect, no rain and personally the crowds were easier to deal with. When people get very hot they get grumpy.
Crowds are usually lighter around that time, because all southern schools are back in session, and it is the start of hurricane season.
Exactly - we hit hurricane season in 2009 and it rained everyday (hurricane Fae)
No matter what, you will have a great time. It's Disney world, how could you not??