Any night that the fireworks are scheduled for, it will be crowded. They say the best place to be is in the hub, but you'll need to be there at least 2-3 hours early to get a spot there. A lot of locals come on Fridays & Saturdays. We were recently there for a Wednesday-Saturday. Wednesday and Thursday were great, Friday was busier (very busy at night) and Saturday (even though it is off season) was very busy. We didn't get much done on Saturday, and we did take advantage of the fast passes for Splash & Space Mountain which had 75 minute waits. Most other waits were 30 min or so.
If DL seems too crowded at times over the weekend, go over to DCA. We were at DCA on a Saturday and didn't have to wait more than 15 min for anything.
