I was very disappointed to learn that on my Disneyland trip in May (23rd-28th), the 27th is a Grad Nite. That will also be our last full day at the park What I want to know is, when do they close off the park?
"Well I don't know about you folks, but it's way past my bedtime."
Grad Nits has no effect (other than no World of Color performances) on closing times in May. Disneyland normally closes at either 8 or 9 PM.
One minor issue, if the park is closing at 9 PM, there is a good chance they will close the Main Street stores at 9 PM, and suggest that you visit the World of Disney store in Downtown Disney if you want to shop for park merchandise.
In June, when the park stays open until 10 or 11 PM during the week, those dates will be affected, as Disneyland has to close no later than 9 PM to park guests. (For DCA, they have to close by 7 PM).