Want to make sure you get an accurate answer here. The more or less 'regular' schedule of EMHs is as follows:
MK - Friday AM EMH / Wednesday PM EMH
Epcot - Thursday AM EMH / Tuesday PM EMH
DHS - Sunday AM EMH / No regularly scheduled PM EMHs
DAK - Saturday & Monday AM EMH / No regularly scheduled PM EMHs
The above is pretty much how it is all year round, with the exception of holidays and certain events where the days of the week may get mixed around depending on the situation. But always check the park calendar because there are exceptions.
There have been periods where Disney has offered more EMHs than the schedule above. Eg, they had near nightly PM EMHs at DAK for around 3 months after Pandora opened summer 2017. DHS saw a two month run of daily AM EMHs after TSL opened. There have been a precious few DHS PM EMHs on the calendar since TLS opening... like only a handful - they are rare. MK will often have daily AM EMHs around Christmas, Easter, etc.
If the park closes at 9pm and has EMH's from 9-11pm, for example - anyone can get in line for any ride right up until 9pm. After 9pm, only EMH-eligible guests can enter a line. CMs will have scanners and/or touchpoints in use to verify EMH eligibility.
That is one of the 'downsides' of PM EMHs. For major headliners that have longer lines still in place at regular park closing, EMH guests don't necessarily have any advantage during the early portion of that EMH time.