Grad nights usually do not impact park operating hours. The event is held in DCA after the park closes to general public at its normal time (often between 8:00 pm and 10:00 pm depending on the night - early closing is usually on weekdays). You can see the hours for
May 2015 to get a sense of when the parks might close next year. Just note they kicked off the summer season on May 22 in 2015 (Friday before Memorial Day) and it might not happen until May 27 in 2016 (Memorial Day is on May 30, 2016). Official operating hours won't be released until about 45-days/6-weeks in advance.
In terms of PtN and
Disneyland Forever (the fireworks show), we don't really know what the schedule will look like for the non-peak season in 2016. Historically, things like fireworks, are only presented on weekends (Fri to Sun) during the non-peak periods. There hasn't been a true nighttime parade in a while, so we really have no recent frame of reference to predict the off-season schedule. DL has been presenting both Disneyland Forever (except on MHP nights) and PtN nightly since they debuted, which is unusual. Normally, fireworks are only presented on weekends (Fri to Sun) and during MHP after Labor Day until the start of the Holiday season. This year, they continued nightly presentation of fireworks. If I had to guess, I think they'll go back to their more normal schedule at some point in 2016, so, to be safe, I would plan on fireworks (and maybe the parade) only being presented on weekends in mid-May.