We went two years ago during the 2nd/3rd week of June and are heading back this year for the 2nd full week. When I was planning two years ago, I was so nervous about the heat and humidity and crowds. Was it hot and humid? YES! But, it didn't bother us too much. We definitely were sweaty, but kept hydrated and took midday breaks every day. The heat ended up not bothering us as much as we thought. The crowds didn't bother us either. Yes it was crowded, but we always did rope drop and came back in the evening. We rode everything we wanted to and more. I am interested to see how FP+ goes for us this year!
If you are planning on going during Spring Break time in (mid to late) March, it will probably end up being more crowded than the 1st or 2nd week of June. And the closer it gets to Easter, the more crowded it gets. But the weather is not as hot.