Its not complicated.
This is how EMH works for all parks at all times (until further notice from WDW).
Look up the POSTED open and close times for any day, any park.
If it is a MORNING EMH, then WDW resort guests staying on-property will be allowed entry to THAT park on THAT day starting ONE HOUR before the posted opening time.
At least one person in your party should have a room key-card with them to verify your current hotel status and gain entry.
If it is a EVENING EMH, then WDW resort guests staying on-property will be allowed to stay and ride attractions at THAT park on THAT day for THREE HOURS after the posted closing time.
(So, YES, if MK is sched. to close at midnight, the EMH will run til 3AM! Yep, that's 3 in the morning.
No, its not a mistake, that would be 3 full hours after the posted time. 3AM for a posted 12mid close-time. 3AM, yes sir!)
EACH adult needs to show a room key-card and a wristband will be issued to each adult and child that the ride CMs will check to allow entry to each ride.
This 1-hour morning EMH and 3-hour evening EMH rule applies at all 4 parks until further notice from WDW.