If the parks are open till 11pm weekdays, then it is definitely Spring Break somewhere. You will stand in line much longer, have a harder time getting ressies, and might be cold, but you will probably get fireworks every night and more park hours.
May is a really great time to go. There is Food and Wine weekend early May. I've done both and we really loved off season. You just have to make sure you get to enjoy the fireworks and ELP on the weekend. We usually don't go home until the park closes so it was nice to have an earlier closing on weekdays to curb the insanity.
I'm going Pres Week break since we bought AP's last May and I'm worried we won't make it back in time. Packing lots of sweatshirts and rain ponchos.