In most situations the application page will open and close automatically. When you create an event, the application page is activated by default, and it will close when the event begins.
There are a few situations when the application page is closed, and here is what you can do about it.
a) If you copy an existing event, you need to active the application page manually in the tab Settings. The reason is to avoid showing outdated information for events that are published as part of an event series.
b) If you want to keep the application page open even after the event has startad, you can set the application page close date manually in the tab Settings. This can be useful when using events as registration pages for other situations than regular events.