This info was specific to the 17th. I'd actually be surprised that they would have high enough crowds to close the parks down. Usually that only happens Easter week, week betwen xmas and NYE. Only other time it happened a lot was when the elecrical parade was leaving park was routinely closed by early afternoon on weekends the last couple months of the parade. Also you can ask as you leave the park if you will have a problem getting back in. If they have are nearing capacity (or have reached it and closed the park) then you would not want to leave between 6 and fireworks. If you were to leave in the afternoon for a break, and come back by 6 or so, I doubt you'd have an issue getting back in.
We go to DL every NYE and 4th of July. Never had a problem getting in. We have left at 6pm on NYE and told we may not get back in. Went and got dinner, were back by 9, no problems getting back in on NYE. Couple of times somone in our party has entered at 11 on NYE. We've also entered up to the fireworks on July 4th.
They usually do a soft closing where they stop allowing certain groups in. Then as people start to leave the parks, they will start to allow more people in, so when you leave you have lost your spot.
For Sunday, the plan is to do a hard close. When they hit the magic number, they're done. Then only re entrys are allowed in from that point.