As far as I know, grad nights are on May 5th and 6th at MK only. That is why the park is closing at 7 p.m. to everyone but grads with special tickets. MGM is closing at the regular time of 8:30, so I am assuming only MK is a "grad night" park.
MK is the only park for grad nights. The grads get to party there from 7PM until 5AM the next morning. The grads will not be at the other parks unless they are willing and able to buy an additional ticket. Since regular guests will not be able to stay in the MK after 7PM on grad nights the other parks may be a little more crowded then usual.