The last week in March is the spring break of the district we are in (it never changes, and we are in the largest district in CO). It seems a large percentage of the families we know, maybe 35%, head to Disney for Spring Break. So, imo, it will be very busy. However, I am also of the opinion there is no slow time anymore. Go in early Sept (when we typically go) and yes, there will be less people but also less staff, and lines will feel just as long and busing will take forever. I really think you just go when it works for you and have a good touring plan.