Here is some good info. From April 18-20 there will be a major Cheer and Dance competition right next to
Disneyland AND the Long Beach Grand Prix. Combined crowds for these two events are expected to be around 350,000! Most of whom will most likely visit Disneyland and DCA at some point during those three days. Combine that with the fact that some schools will still be on spring break and you have an EXTREMELY CROWDED park.
I was at Disneyland last year when this same cheer competition was going on and it made the parks the most crowded I've ever seen...the lines were just crazy (about 2 hour waits for almost every major attraction.)
If you are absolutely locked into going the dates I mentioned, then I strongly suggest visiting Disneyland on Friday and DCA on Saturday to avoid at least some crowds.