Mornings are always light until 10-11am or so, then all the families start piling in with their kids. Normally the park starts emptying out around 4pm as those same folks head back to the resort and/or go eat. But on nights of the party, you can go in as early as 4pm. Those with a regular ticket and a party ticket can just stay in the park, but you'll still get an influx of party-only ticket holders.
If you stay for the duration, it often (but not always) lightens up, sometimes considerably so as time goes on. I love this time myself, as I'm in it from opening to closing - and going on rides like SM as fast as you can walk/ride the loop is neat. Some rides, like BTMR might have only a few people in the queue and you can just stay on to ride again.
On a day with the party, you have people that have tickets one way or the other for the day - and others that only have party tickets. The former will get there as normal and the latter not until 4-7pm. So no real effect in the end, I'd say. The exception would be those that have both. They might come in a tad later, but chances are these types of folks (like myself) are there for the whole shebang.