Making mobs continuously spawn near player?

[Eria8]

I want to make a sort of event where lots of mobs continuously spawn near the player one night until dawn.

 

I was thinking about having a chance to start the event every time the world time hits 8:00 PM and every few minutes check if random coordinates near the player can have a mob spawn there and then spawn a bunch at like 20 of those random coordinates.

 

But that doesn't seem like the best way to do it. And I'm not sure how that would turn out on a server.

Is there an easier way of doing something like that?